If you are interested in playing Standard, there is a rotation coming in the Fall. The Sets that will rotate out at that time are as follows: Kaladesh, Aether Revolt, Amonkhet, and Hour of Devastation; so I would suggest not buying too heavily into any of those sets.
www.mtggoldfish.com has a pretty well maintained decklists/metagame section where you can look at what decks are popular in each format, to try to find something to fits your tastes. It also has budget deck sections for if you just want to dip your toes in and see if you actually want to go further.
The most popular constructed formats these days are Modern (All cards from 8th Edition Forward), Standard (Yearly Rotation, 5-8 sets in the format at a time), and Commander (Casual multiplayer format, 100 card singleton with special rules). Block constructed has been fully retired. Legacy still exists and is supported, but is well out of your listed price range.
My suggestion with 250 bucks would be to go to mtggoldfish and look at the metagame decks for standard or modern (although you're less likely to find an affordable one for modern, due to expensive land concerns). Many of them will have play-footage linked at the bottom of the page so that you can get a feel for how they play and decide what deck you'd want to give a go. Then, I'd settle on a list that's hopefully not too balls-deep on the rotating sets, and go to TCGplayer (american) or Magic Kard Market (european) and buy the specific singles you need to build the deck/s you like. Straight up buying booster boxes doesn't really give you things you need for play, at least, not without throwing a silly pile of cash at it. Alternatively, if you like buying stuff from your local stores and don't like singles shopping, Challenger Decks are coming out next month, which are off-the-shelf Constructed Standard decks with pretty good value in cards. They're about $30 each and give you a solid foundation for a decent standard deck. The one iffy part of the Challenger decks is that they contain a lot of cards that'll rotate in the fall, but if you just want to have quick compact fun for the next 4-5 months they fit the bill. Of course, with a new set coming out in ~3 weeks, it's anyone's guess what will be strong then
The Black/White Vampires deck that's been doing okay recently might be a good conservative investment, as it relies on few cards from the sets that will be rotating in the fall, and of those cards, the most expensive are lands that will likely retain their value or be easy to trade/resell.
There's also Commander and the upcoming Brawl formats that are potential cheap casual play options, but that depends heavily on your local playerbase. Brawl is brand new, so there's no way to know if it's going to be popular, and Commander's powerlevel is wildly variant from group to group. In one group, a $70 upgraded preconstructed deck is a good powerlevel, in another group your opponent will throw a bunch of moxen and timetwisters at you first turn and your wallet will die. If you decide to Draft in standard as your way of collection building, Brawl will naturally make use of your excess draft chaff, since it's restricted to standard-legal cards.
Another option that will open itself later this year is MTG Arena, Magic's new "free to play" digital platform. This is pretty much strictly Standard/Brawl/Limited focused though, and the economics of it are still being hammered out.
So that's the economics bit.
Gameplay:
Main changes since 15 years ago would be:
-Mana Burn no longer exists.
-Damage does not use the stack. This has made creatures with sacrifice abilities a bit worse, but has improved the gameplay tension of making choices with said creatures, unlike before, where the correct option was always "sacrifice with damage on the stack."
-Planeswalkers are a new card type added to the game. They represent fellow Planeswalkers who are helping you out for a time. They enter play with a certain # of "Loyalty" counters, and can be attacked by creatures and shot by burn spells that target them, losing "Loyalty" in much the same way you lose life. Each planeswalker have a series of Loyalty Abilities, of which they can only activate one per turn. Usually, one ability increases their loyalty for a small effect, another ability decreases their loyalty for a medium effect, and the third ability decreases loyalty a *lot* for a big effect. You discard the planeswalker from play when its Loyalty hits zero.
-The Legend Rule changed twice while you were gone. The current form of the Legend rule is basically: if you control two Legendary Permanents with the same Name, put one of them into your graveyard immediatly. You and your opponent can both have the same Legend in play with no consequences.
-Generally, Creatures are much more powerful than when you played, and Instants/Sorceries are generally weaker. Face burn has been reeled in significantly over the last few years, as have Counterspells.
-A new level of rarity was added to the game; Mythic Rare. There are roughly 15 Mythic Rares in a set, and you average about 5 Mythic Rares per booster box. Planeswalkers are always Mythic Rare, and the general philosophy of Mythic Rare is supposed to be "super sweet splashy cards," but sometimes they go for selling packs and greed and just slap a cheap efficient constructed staple in there
-Colorless Mana now has a Symbol, instead of sharing the number-in-a-circle symbology of Generic Mana (which is still around, and is what shows up in costs). The new symbol is a Diamond-shape with gray backing.
-Combat has had some adjustments that only really matter in extreme corner cases, but it'd probably do you well to just read the rules entry for the combat phase so you don't get caught offguard when someone does something in a step you didn't realize existed because we still shorthand things the same way we did before
-Equipment, a new artifact subtype was added in 2003, so i don't know if that was before or after you quit, but it's basically re-usable Enchant Creatures (which are also now called Auras), and mostly sucks balls because it's hard to design balanced ones.
-Vehicles, a new artifact subtype was added a year and a half ago. You tap creatures to "crew" the vehicle, turning it into an Artifact Creature and letting it attack/block/do shenanigans. They are very similar to Equipment, and mostly bad.
-A lot of random terminology adjustments. "In play" is now "on the Battlefield," because folks got confused between "playing" a spell and having a card "in play." "Dies" is shorthand for "moves from the Battlefield to the Graveyard." Remove From Game replaced with "Exile." A new ability, "Lifelink," has replaced the old "Whenever this creature deals damage, gain that much life," on newer cards, but is mechanically distinct from the older cards with that ability.
-Mulligans have changed. When you mulligan, you shuffle your hand into your deck, then draw Cards equal to the number of cards you shuffled in, minus 1. You then decide to keep or to mulligan again (losing another card if you mulligan again). Then, once you've settled on a hand, if it has 6 or fewer cards in it, you get to "scry 1", which means "look at the top card of your library, and choose to either put that card on the top or the bottom of your library." Makes the first mulligan a lot less punishing.
-Glorious Lands-In-Back Partisans finally won the long and bloody Lands-in-Back vs Lands-in-Front war.
