Key question - how long do you plan on keeping it, and how long will it survive. The thing that tends to kill cars here is rust. If it is solid then you fix the other stuff and keep it going.
Lets say these repairs cost £300 but you get 2 years out of it, thats ~£12 a month in parts the car owes you. I would bet you would make that back in fuel saved over running the 328 (or even the 318iS) every day. Chop one of those in and the gains get even better.
More money saved = more parts for a faster 535i.
