I would say £9k.
There are some sub 60k mile 335i coupes on autotrader at dealers for £11-13k.
So being realistic the Cat D makes it worth around £6-9k.
The M-Sport kit will make it more desirable though and push it to the top end of that I would guess.
Only problem you may have is insurance and finance can be a tricky, finance because it is a cat d and insurance as it is heavily modified.
It needs to be declared as an SE with loads of extras.