You should buy the game, not download it .. it's not that expensive
But to answer your questions, yes there is an anti-aliasing mod:
The episodes for Gta IV (TLAD and TBOGT) work with the same files mostely, weapons , player items, car mods work on every version of the game so it doesn't matter if you use them for Gta IV or the episodes. Script mods are the only exception, some need a specific patch to work but most will work with Tbogt/tlad too.
Prepare yourself though, because modding means backing up files (mods replace default files mostely) so that means you'll have to backup your entire game. If you really can't spare the memory then you can also backup files you are going to replace but sooner or later you'll mess up and not have a back-up if you do it that way. That would mean re-installing the entire game to get that default file back.