I know there are plenty cars on here running Porsche wheels, i'm looking at getting some Design 90's for my Fabia, i know it will require hub adaptors but my question is how wide should i get the adaptors, i see they come in various sizes, i was thinking of getting the 20mm ones but would that be enough or how much wider can i go without any problems?

the wheels themselves are

Front: 16x6J

Rear: 16x8J

Depends on the offset of the wheels but i'd say 20mm would be fine.

offsets are something i've never managed to get my head around but i'm guessing this is the offset?... ET52.3

another question, i've only found H&R hub adaptors which are at least £80 a pair, what other ones are out there and any idea of prices?

£80 a pair is about right for adaptors.

Yeah, for Porker wheels you'll need 20mm adaptors to bring the offset into a usable range

how much extra strain do they put on the wheel bearings and other components?

none, its the relative offset of wheels that puts strain on things not using spacers as such.

if you use 20mm spacers on the correct offset then strain will be increased but if you fit 20mm spacers and bring a wheel into the correct offset then it will have no greater strain than standard wheels.