hmm, you would have been better off doing the mods to fit the wheels and tyres before painting. But it can be done without cracking the paint if there's no bog around the wheelarches. I did mine myself, but if you haven't done it before and are worried about a new paint job, then you're better off paying someone to do it.

15x8 under the front will be a very tight squeeze. Yes, more positive offsets bring it closer to the strut. I don't think you'll be able to fit anything other than the 0 offset on the front on standard struts and brakes - even that is going to be pretty tight to the strut but should fit IIRC. I'll be changing my front springs this weekend so i'll whack one of the 15x8s on the front, take a photo of how it sits and and measure clearance to strut for you. Front gaurds will need a fair bit of work to get 8" fronts under too. Rear was pretty easy by comparison but still took a fair bit of beating :) A 4dr sedan might have a bit less room under the rear than my car, so you really need to decide for yourself.

All the clearances to the strut and body depend on what struts, brakes, camber tops, diff etc you have on the car though.