1) 100 meters max. With consumer equipment I'd stay under 50 meters just to be safe.
2) most Cat5 I can find around her is either "general purpose Cat5" or Cat5e, which is higher quality stuff and not really much more expensive. Cat5e is useful up through gigabit ethernet speeds so you don't really need the more expensive Cat6. But be careful of where you install cable-if it's going in an air space that might be part of the room air space, use "plenum" cable. Plenum cable has a jacket made of a plastic that does not create toxic vapors when it burns, while non-plenum cable is typically jacketed with PVC-a nasty stuff to have in a fire. There's also outdoor-rated cable that has UV-resistant jacket material, but that's getting off track...
I recommend Cat5e plenum-rated cable for everything. It handles everything you need it to, it's safer, it's not significantly more expensive than the cheaper stuff, and having only one kind of cable (whether you're installing in a plenum space ro not) makes things simpler.
Helpful?