To add to that, you definitely need the expansion, as the base game is very incomplete nowadays with most of the current content being lvl 70.
You can find cheap keys for the game on various kinda shady key sites like G2A and cdkeys and whatever else, google will find you some for sure. Total price is like 20-25$ if you do it that way, and the keys should work fine, the lower price comes from other markets and physical boxes, but there's always a small risk of getting fucked when using these. Otherwise you have to wait for a rare Blizzard sale, and even then it's still that same price.
As for is it worth it, I think it is, it's a really fun game especially if you haven't played it yet/in a long time. It doesn't have as much longetivity as say, PoE, due to a more limited scope in builds and stuff, but it's still very enjoyable to play once in a while when they start a new season/release a new patch. The season journey gives you clear objectives to aim for rather than just farming for farming and the many added cosmetics also give you stuff to specifically farm if you want to. There also isn't really a barrier of entry like in a mmo, due to seasons being reset every 3months or whatever, you can just pick up the game and never be too far behind. Might be troublesome to find groups for greater rifts late in the season if your paragon is low though, but you can kinda level that on your own fine since solo xp is pretty good nowadays. In general soloing is perfectly fine, I personally don't like grouping much in ARPGs and I play D3 every season and have no trouble completing the season journey very quickly. The main concern is if you wanted to compete on the leaderboards, since most ppl on solo leaderboards farm in groups a lot of paragon levels and gem levels before doing it, which massively skews the odds in their favor, but leaderboards are for ppl who play everyday the entire season anyway.