There’s a useful calculator over on
www.willtheyfit.com - if you input details of your existing wheel/tyre set up and the set up for any new wheels/tyres you’re considering, it’ll calculate the ‘vital’ numbers, such as changes in clearance, speedo reading error etc.
The spec for 18” factory fit alloys for the Leon is 7.5J x 18 ET51 - centre bore 57.1 and PCD (bolt pattern) 5x112, with 225/40 R18 tyres. In your position I’d aim for something that’s close to the factory spec. I’d personally avoid diamond cut alloys as the diamond cut finish isn’t particularly durable - especially during the winter months. They’re also more expensive to refurbish than alloys with a painted/powder coated finish, and fitting larger alloys could increase the risk of kerbing your wheels. If you did go for diamond cut, I’d suggest keeping your existing 16” wheels for winter use to
help preserve the diamond cut finish.
Agree 100% with
@Damo H about tyres; always go for something decent. Of the premium brands, I’d avoid Bridgestone though as IMO they tend to be quite a noisy tyre, and they’re manufactured from a ‘hard as nails’ compound, so wet weather grip isn’t great either.
I’d generally avoid cheap Chinese imports, as the finish might not be great, and there’s a good chance they’ve not been subjected to the stringent quality and safety testing and certification that wheels from reputable manufacturers are subjected to (TUV, ABE, JWL or equivalent). If wheels have been quality and safety tested, then they’ll have the TUV/JWL etc. safety mark forged into the back of one of the spokes. All other relevant information such as wheel width, offset, manufacturer etc. should also be forged into the reverse of the wheel spokes. If you’re considering genuine Seat alloys, then the Seat logo and part number should also be on the reverse of the spokes.
If you’re considering second hand alloys, always check with the seller that they aren’t buckled or cracked and haven’t had any structural repairs that could compromise the strength or safety of the wheel.
Also, if you fit new alloys it’s classed as a modification for insurance purposes, so don’t forget to inform your insurer.