A lot of people have success with Paasche and Iwata brushes. Airbrushing is tricky to learn at first, and a quality brush will help minimize frustration. Most artists recommend a double action airbrush. I adore my paasche.
Compressors are similar. I tried to go get a cheap basic air compressor from Walmart. They pressure was too high and hard to control. I took it back, saved my money up and got an iwata double chamber compressor. It allows me to control the psi (for ponies I prefer 15-30 psi).
Sculpting I would recommend apoxie sculpt anyday. They sell it on eBay, or their website,
www.aves.com. It's a tad bit pricier than other clays, but it air dries and is wonderfully rock hard when it dries. It's a two part clay that I adore.