The ESVA requires a model-report and is very strict.
Auction grades is a pretty good pointer to the cars overall condition.
Grade 4 with interior grade B is usually very good.
But a grade 3.5 with interior grade C could also be good. I was recommended to go for a grade 4 with interior B and I was more than happy with the condition of the car.
There are grade 5s out there too which is as good as it gets considering the age of the S15.
They're pretty rare and usually very expensive, so not really worth it unless you're very very picky (psycho style).
have a look in the useful threads section in my "import story" thread. Lots of links and info there. The auction grading translation from GBautotrader in that thread is very harsch though.