what size are they now? i'd guess 13x5.5? thats what my seat 13's are?
I would say you could go up a couple of sizes. on 13's you should be able to get 7's in ok.
I had 15x6.5 on my mk3 and they were fine at the back, but touched the inner wing a little on the front. which was fine as i just gently bent the offending piece to the side by hand.
Tyre wise, you'll need to drop a size to get straight sidewalls, i.e if they should have 195/55/13 I would go for 185 or 175 depending. they wont look stretched and wont roll so hard in the corners. Will give you a bit more clearance on the outer arch lips.
Best bet is to put the wheels on and measure how far from the arch lip they are, then that tells you the max you can go out. Remember the offset too, as most banded wheels only add to the outside of a wheel which pushes the offset away from the et38 it should have.
I would tap the lips over in the back before you paint. Theres no reason not too and if done slowly and gently you'll gain about half an inch lmao
How are you lowering it? if it's just springs it'll always rub and may well bottom out and wreck the rears.
Make sure you get a set of matched shortened and uprated dampers along with the springs.
Jon.