hi if your buying private then I would HPI the car, the 335 I just brought for my mrs had 9k outstanding on it, however the chap did tell me that but its always better to be safe then sorry. Check the SH if its all from different garages then I would question myself why (I assume its only had 1 - 2 owners) so if its had a few services all from different garages then it may raise a few questions.
Make sure all the electrics and toys work, check panel gaps paint work etc etc, tyre treads. If the tyres are some un known brand from china then what else hasnt the owner spent money on? Are the tyres matching pairs?. Go through the idrive and see what is due to changed in the near future as these cars are conditional based service
Also take your car insurance details so you can drive the car and take it for a drive on different roads, motorways, B roads etc etc, keep the radio off so that you can hear any bumps rattles etc etc. How doe sthe ownwer drive the car if its cold and he steps on right away then maybe he always drives the car like that, the worst thing they say is 'it goes well doesnt it?' real put off for me
There maybe a guide on this site for specific problesm relating to the 335 so it could be worth trying a search. buying a car should be enjoyable not a stressful event :-)
Id also take another person along 4 eyes are better then 2 lol!.
let us know how you get on.