acquire the SSI and Microprose libraries by hook or crook...
Darklands alone would be worth it, not to mention the entire AD&D gold box series.
Acquire rights to Bioware's Neverwinter Nights and, more importantly, their old DLC modules as they're not available ANYWHERE ATM...
Infocom adventures, bonus for porting Quarterstaff to DOS/Windows or branching out into emulated games beyond DOS, and picking up some of the other older Macintosh RPGs that have, essentially vanished, e.g. Taskmaker 2. (I slogged through the first one as there weren't many Mac RPGs, but ENTIRELY missed Taskmaker 2... probably not worth it(game was HORRIBLE), but...)
Don't give much of a rat's ass about old strategy, FPS, hunt-teh-pixel-adventure, etc. though.
[EDIT]
Even though you're a bunch of Poles, stay the hell away from Eastern European titles as they mostly suck alot! Especially stay away from 1c garbage as it's just garbage.
Yeah, The Witcher wasn't bad, nor was Two World, but the rest was shit, to put it mildly.
[/EDIT]
[EDIT2]
This is not to say that I,personally, would buy any of these titles as I already own virtually all of them, individually and in collections... but some of your customers MAY find use for having the titles available digitally...
Just please, truly remove DRM on these game though. Ok? It's not hard, hex editing and/ordisassembly will get you there... then go back and do the same on what you alreayd have instead of lying about being DRM free...
or failing that just pick up the cracks...
[/EDIT2]
[EDIT3]
As to crappily buggy Eastern European games: well if you actually fixed them some of them might actually be fun, e.g. Not the Time for Dragons, Planet Alcatraz, etc... Might also help 1c realistically adjust their income expectations from old games...
[/EDIT]

EDIT : I just realized who posted this...kind of explains the ridiculous crap that made up most of the post. Ugh. I wish I could delete this reply.