You'll probalby need to stiffen up your drivetrain and suspension if you go down the route of slicks fella. With all that extra grip, something will go pop under load.
It may also be worth getting a set of old, used 15" rims, by the time you've bought 17's all round you'll have probably spent the same and your next set of slicks would cost less fresh 17's
Why do you need slicks?
You obviously don't need them for on track, so just curious.
I wouldnt buy second hand slicks personally as they deteriorate with how many heat cycles they have done and its not easy to tell just buy looking at them.
Whenever we have bought them we went through the manufacture.
They may be expensive, but that's to be expected, they're the opposite end of the spectrum to BlackCircles Ditchfinder Pro's.
They are expensive yes. We were paying £400 a corner I think in the Xbow, not sure what size you will need tbh. Slicks are completely different to any road/track tyre, to get them up to temp u have to be aggressive and be prepared to slide the car a lot when they are cold.
We bought our slicks at circuit from the manufacture but we also had some delivered to the workshop from Avon and Dunlop.
In my opinion you only need slicks if you are racing & or a driver with an ability where you can explore the limits of the tyre. Being a slick it's only purpose is TRACK. Having extra grip can cause problems as above... For instance it'll be harder on your whole drivetrain/Wheelbearings/suspension for a start.
We were running 18inch wheels with a not very popular size so Id imagine yours will be cheaper.
Clio cup slick will be the right size and can be had for £30 a tyre part worn or around £200 new I've been running them for years
Clio cup slick will be the right size and can be had for £30 a tyre part worn or around £200 new I've been running them for years