As said previously, lower your car on the 17's and then go from there.
You said you want it to be low and slow too - you can't really get that low on 18's!
My old Mk3 was lowered approximately 100-110mmm (compared to a standard height Mk3) on 17's and I think it looked pimp as fcuk.. handled quite well too!! But the sills and splitter still weren't low enough to the ground for my liking.

On 15's, it was pretty much on the floor and I found going over speed bumps to be a complete nightmare cus I just beached it all the time, although it handled 100x better and I enjoyed it a lot more

And my new car on 16's is just perfect all round in my opinion
