What do you want to do with the car?
Cheap tyres are cheap for a reason - and one day will bite you hard when you least expect it.
£50 tyres are going to be bobbins mate - go for some toyo proxies, I can recommend the Continental ContiSport II's - they grip they don't wear too hard - they will last and you wont end up up-side-down in a ditch.
You will get what you pay for ultimately - expect to replace them after a few thousand miles.
Oh and if you're looking for centre caps for 18" TTE's and you got them from eBay there was a thread on here about them and they looked a bit dodge - have they turned up yet? I could be wrong mind.
Seriously though fella - don't skimp on tyres, you've got a Lexus, not a 1 litre F Reg fiesta. What is the speed rating on the cheapo's you're looking at?
Sorry if I seem like I'm ranting fella - just trying to help you out here.