There's a pretty good gun on sale now, $30 off, here:
http://www.caswellplating.com/powder/index.html
I started with one almost identical to this one and it worked really well, especially for the price.
If you are talking about the one from one of the department stores that use to be big in catalog sales, I see a lot of those on ebay that have "only been used once" which tells me that the seller might have been disappointed with it.
Cleaning and prepping: You might get some answers here but if you use the search function and look for words like "prep" and "cleaning", you'll find a lot of discussions have already taken place and you can get faster answers. I'd give you more info but I'm one of the guys who blasts, blows the dust off, and coats, which isn't that super of a method but seems to work.
Steve