RMS is Rear Main Seal, were oil leaks and cause engine failure. It is a design flaw in the boxer engine. The Boxster is very easy to work on and do stuff to, but I would beware of any engine type fixes. The engines were not made to be rebuilt but replaced which can cost quite a bit. They are plenty reliable if you drive them, but I would seriously consider buying a high milage '97 for 10k before I started looking for a rebuild project that could cost thousands.
Now if your are looking at '00's with 20-30k miles your should be fine with a good PPI before you buy. Most RMS failures mostly occur before 25k miles, and more often in older cars, but it is random, so don't hold me to that. If you take care of the car and drive it you will less problems than if you let the car sit a lot.
There are a lot of sites on what to look for just do a google search.
Good Luck finding a Boxster!
