You do not need 700 NES games. 90% of the NES library is either shovelware (terrible movie tie-ins) or brutally difficult Japanese RPGs that haven't aged well. You will download a 500MB set, scroll through a list of 700 games, play Contra for five minutes, and turn it off. You are suffering from "ROM paralysis."
: Tries to include every known version of a game (including bad dumps and hacks). This set is huge—over 22,000 files. link download complete nes rom set
For $20/year, you get access to a curated collection of 100+ NES games (with more added regularly). These versions include save states and online multiplayer. It is not the full 700 games, but it has all the heavy hitters. You do not need 700 NES games