One of the lads in work has a e36 325 so I can give ya some info.....
I think it's a H reg, older than mine by 2 or 3 years anyway.
Do they have any common faults?
Don't think he's had any problems with it at all, so can't help ya there much, sorry.
Do they rust badly?
The arches are rusted quite bad. Other specs of rust appear round edges of the bodywork like doors, bonnet, sunroof e.t.c
Are parts easily come by,and do parts cross from model to model
I'd say so, There are plenty of spare parts, both scrap and new'uns on ebay alone. Best do a quick search on there to see how easy or difficult it is to get hold of parts for that specific model, that specific year and that specific engine type and size - thats the difficult part. Although, when I search for parts for mine, I get quite a few 325 parts aswell.
Hope that helped, probably not but oh well