I don't think they'll let you convert 50 tomes into 60 tomes, but maybe. The conversion rate is shit though, and it'd convert into the lower tier tomes, not the high tier, aka tomes that aren't capped and you can get running shitty 60 dungeons. Like in current game terms, you'd get soldiery, not poetics. And only like 400-500 of them too.
I mean maybe they'll do it differently, maybe poetics will be the new tomes so having 2000 would give you a slight edge, but eh, it probably wouldn't be that much.
You could farm a bit of money for armor yeah. Though armor isn't super important while leveling. Get a chocobo to 4 in case you have to solo stuff for the heal, and armor you upgrade every 10-15 levels. Jewelry you upgrade like twice(lvl 1-10 jewelry then you upgrade either at 28ish or 35ish, and then keep that until current cap then get whatever from quests in Ishgard). It's the weapon you really want to upgrade often, sadly since it's new weapons you'll have to find someone who can make them, or get your crafts to 50. Blacksmith or Goldsmith to make guns probably, though Alchemy is also a possibility. I'd consider getting these to cap using dailies(GC and Ixal) and/or leves.
One thing to note as a returning player, doing story quests yields a few top tier materias at certain steps(iirc leviathan quest and shiva quests) which you can cash in for like 400-500k each, though that depends on server(it's crit/det IV materias). So you should get a decent bit out of that, enough to buy a bit of gear or what not. Also get a crafting, gathering and quicktongue IV and some random food, so it's like 1.5millions total or something.