You can regulate the output air pressure - they don't have to work at maximum pressure. For blowing out water lines you want air volume. Five gallon tank, at least, and a 5 hp motor would be good. But I didn't do that.
I bought a Deltal oil-less compressor - http://www.deltawoodworking.com/index.asp?e=136&p=4722
. It will inflate the tires and, being oil-less it will not introduce traces of oil into the water lines. I got it at Lowe's for, I believe, $89 a year or so ago.
HOWEVER, its chief attraction for me is that it will store away under the wardrobe floor in the trailer. That way, I can blow out the lines at a campsite (or top off tires) before returning home in freezing weather. If I was not going to carry it with me I'd want something with a larger tank for blowing out water lines. It does the job, but slowly.
Oil-less compressors are somewhat loud. If you want a quieter oil type you can get the filters made to keep oil out of paint guns and so forth.