400 sounds a lot, but if its good....
oh and you dont change the oil filter on one of these babys, it has a centrifugal filter which you just clean and put back, fairly simple job. Im a BIS owner myself so cant be of much help with aircooled models but i can tell you to inspect the bodywork for rust in all the usual places.
Which production year is it by the way, is it an original Italian fiat or a Polish FSM? id love to see a few more pics if you have.
Cheers,,,