Don't buy cheap spacers
And i would only recommend hubcentric ones, you can get wheel wobble, uneven tyre wear etc from ones that don't sit on the hub - the weight of the car is only supported on the bolts otherwise
There is chap on Ebay - ESP engineering, based in Crowborough, who makes excellent quality ones...
As far a show big a spacer you can fir depends on the ET of your wheels
mine are ET31 and i run 15mm spacers each at the rear and the arches don't catch as they're rolled and pulled slightly