Powder coating is done in a oven at 185C, so will not affect strength etc of any wheel.
Powder coat can be done is different colours on the same piece, if you can find someone who can be bothered as keeping the powder colours seperate is a pain, and difficult as you can't do a sweep half way through.
Metallic CAN be done by adding particles before full bake has occured, ie when it is semi liquid and tacky as hell, these will need a sealing powder(clear) applied afterwards though so cost more.
Most decent places will chemically strip first any wheels etc being done.
Good places to check are places that do their own stuff in the colours you want, as it is only a case of jigging it up to do it.
Ie if you want red got to fire extinguisher manufacturers!!
tabetha