dont worry about all that offset rubbish. i was desperately looking for a set of focus RS wheels to go straight onto my mk1, but got put off by 2 things(even the price i was prepared to swallow(big bucks)). the first was the offsets, which like you i havnt really got a clue about and the 2nd was rubbing issues, whether that was because of the offsets or the extra clearance because the standard focus body differs from the RS. i still think the RS looks special, but as the focus is my missus's car it wasnt really worth it, but i still want it to look good, so i went onto ebay and got me a set of used wheels and tyres from an st170 which bolted straight on. i had to spend a little on a couple of tyres which was a good job because there was 2 that were strangely narrower than the standard size for a st170, 205 instead of 215!!??

i tried also to get a set of MP3 wheels off a focus sport, but these are rarer as i just guess st170's are crashed more often than sports.

dont worry too much about offsets, just go and look to see second hand st170 or MP3's, just takes a little patience.

tyre size will be 215/45/17, with a pressure of 34psi which gives a good confident ride