It seems like alot of problems to keep her running! Unless you have some serious modifications to the car I would suggest selling it.
On the other hand if it meant holding on to it untill you have enough money for the M3 then you can get by with it unless the diff is a complete headache!
Also there are a few places in the UK that will completely swap your engine and replace it with a fully reconditioned engine for around £1500
But if it were me I would just let her go and go get the m3 slightly earlier than planned and save yourself money in the long run.