The problem you will find with a lot of the after market calipers like wilwood, harrop, AP etc is that they are designed with larger diameter rotors in mind. Fitting them to a smaller rotor makes for problems with the different circle radius of the caliper to the disc and the pads don't fit the discs correctly.
If you do fit an after market caliper look for one that suits the disc diameter you intend to use.
I'm using Hi-Spec calipers and they seem pretty nice. They are machined from billet and designed for UK road conditions. External dust seals make them ADR legal too.
Their Monster range has a 4 piston caliper designed for a 300mm rotor. This should fit inside a 15" wheel OK.
http://www.hispecbrake.co.uk/calipers/M4%20-%20300.htm
They also make them to suit pretty much any thickness rotor.
Cheers