Should I fit new driveshafts to my build, New complete shafts are £75 each. Or should I pay £30ish each side in parts, strip, repaint and rebuild?

if the £75 includes all C.V. joints and are ready to fit, I would buy them and save the time on refurbing the old manky ones.

Drive shafts are pretty robust, i just painted mine and fitted, thinking that if i came across a problem with them i could fit another set any time

New shafts are £75 each? Thats VERY cheap. Replacement CV's are more than that and theres two of them..

Go for it I would say, if you dont have the ones from the donor?

Yep...

http://www.eurocarparts.com/ecp/c/BMW_3+Series_328_2.8_1997/p/Car-Parts/Transmission/Transmission-Parts/Driveshafts/?654113161&1&aa9eae49f9f7b b6030904caa34d7219398996f4e&DRSH

The ones off the donor will not fit as I'm not using the m3 brakes..... 315mm ish front and 286 rear might be a tad too much

The driveshafts aren't going to be subjected to the forces they were on the original donor vehicle, so I'd save a few quid and refurbish some used ones.

Phil