Being awd, you will have to keep the rolling diameter the same for front and back or the attessa will trip out! Which makes stagered tyre sizes very tricky, probably best to use the same front and rear! The offsets wont cause any problems, but stags have more room in the front than the back, and 10" will be a very tight fit (will need guard rolling)! *I just picked up 18x9.5 +14 and they dont fit, going to need guard mods (would be the same fit as a 10 +20)!
Im currently using 18x8 +35 with 235/40 all round, and the back fits ok, but the front are swimming in the guards; so a 9 +20 would be a good straight fit (using same tyres)! Id personally use the same front and back! Im about to put the new rims with 255/35 on, but doing some serious guard work to fit them (rolling dia. is about the same as factory)!
my 2cents...