zero. no games at all will work flawlessly. However, in a large proportion the average person (without access to a backup unit and some understanding of emulator internals) won't recognise the flaws, and a large number of the remaining are still fully playable. So you will get many different answers depending on an individual's standards. If you are a perfectionist/unusually fussy, i suggest buying a real n64 system. Actually, i suggest buying a real system anyway, so you aren't being immoral. Over 80% of games work well out of the box, the problem is when someone wants to play e.g. Resident Evil 2, Pokemon Puzzle League and Hey You Pikachu, for example (which seems to happen quite often?). So look at the games you want in the ROM Browser, note the Status (colour) and decide for yourself if it's worth investing further in this.
As for the second part of your question, speed, that's a matter for the host system - your system ought to be able to run everything at or above full speed i think.