Hello Penn0r. ?Welcome to the site.
You can stick 18s straight on your golf with no adjustment required. ?Make sure your new alloys have got the right bolt arrangement and offset ... 5 x 100mm bolt arrangement, -30i to 40 ish offset, I think. ?(I'm sure someone will correct me here if I'm wrong).
For your new alloys, in order for your speedo to stay accurate you will need to make sure that the overall radius of your wheel is still the same. ?For 18"ers you'll need 225/40/18 tyres, and for 17"ers you'll need 225/45/17 (this is what I've got on mine - see pic below).
I assume you mean that the tyres will be fitted on the alloys in the shop? ?You can easily fit your wheels yourself - just swop em over, like your putting the spare wheel on!
As for suspension, you don't NEED to do anything - the arches on your golf will accomodate the new wheels fine if you stick to the tyre sizes I mentioned above. ?However, with big wheels it will emphasize the fact that your golf is a bit jacked up ( as Kniterider on this forum once wisely said, "it will look like an oil-rig" ). ?Therefore, I recommend lowering your car by 30 or 40 mm. ?Mine's dropped 35mm from the original GTI spec (already lower than yours will start off) and still doesn't look that low.
Hope this helps ?
