Getting a good job with powder really depends on who you use. You don't want a big outfit that usually does thousands of parts and will fit you in. What you need is a job shop. Someone that will work with you and help you get the finish you are looking for. Powder is available on many colors, but if its unusual, it usually requires the powder coater to purchase a box of powder just for you. For special order colors, expect to pay a premium, as small quantities of powder cost more than what he pays for colors he buys in bulk. Plus, it requires setting up & cleaning the gun just to spray your color. Again, you will have better luck at a job shop getting a special order color done. Some colors require a clear powder over the base color, just like paint. There are specialty powders that are better for chemical resistance, salt spray or UV, etc. For an indoor displayed pump it may not be critical, but if its going to be outside, be sure and tell the shop you need a powder that will stand up to the weather. Even if you are going to paint, the powder makes a very good primer coat.
As with most everything, some places do a good job and some others, well you know. The metal must be absolutely clean and have no paint, etc. Most places sandblast or maybe chemically strip the metal. Steel takes powder really well, some aluminum castings will outgas, but that is not too much of a concern on pumps I don't think. Powder can scratch so I wouldn't buff it, probably furniture polish or the like is better. If you only have small imperfections, you can sand and recoat powder much like lacquer paint. Another thing to keep in mind is powder is much thicker than paint and in areas where clearances are critical you may not want powder there. Something like a pipe that fits inside a hole or maybe the hinge pin.
I'm on my first pump and I will be taking the frame and base to the powder coaters real soon. I will definitely be doing the outer panels as well when they are ready. I have had numerous car parts and even bicycle frames powdered, its great stuff. Heck, I even had the satellite TV dish mounting pole powder coated! If you can't find a good powder coater in your area, try getting a recommendation from a custom motorcycle shop. Those guys have a lot of stuff done and are generally pretty picky!
