I want to replace my standard back-box on my 1.4 CL mk3 but I'm not too sure what I need to look for when buying a new one..

Will most/any universal fit or is there something special I need?
What size is the pipe connecting to the backbox on a mk3?
any loss in performance if I get a cheap one?...
No laughing at my idiocy I'm new to this

Thanks guys!
also a quick question about the front indicators and blanking covers on the front bumper. I brought some lens tinting spray and wondered how I can get the len's off of these parts.. Will I just need to prize them off and then stick them back on with some form of sealant?