Choosing the right powder coating gun

Well you do not want a gun that is sold for the hobbyist (cost about $90.00 or the set up that costs $10,000.
I would imagine that you will be involved with several colour changes in a day being a job shop, so quick colour change is important. Spray from the box (sometimes called dip stick method) would be my choice (10-15 mins clean down). A gun with variable KV and air control will cover most of your requirements (second coat PC requires lower Kv). Technical support and ease of obtaining spare parts would also be high on my list. As you are aware there are two types of PC guns ò Corona (most popular) and Tribo (for big industry). ITW Gema and Volstatic have been around for years and there are some good PC guns from India (we use one in our labs) that appear to work satisfactory and realistically priced.
As a job shop you will spray to waste so no need for an expensive reclaim system. I would think spray equipment in the $1200-$2500 range should be sufficed for your needs.

If you are only looking at the charging method, corona for instance, it all works the same. What you want to look at is the powder delivery method, ease of cleaning between color changes, pumps, hoppers, accessory selection in case you want to upgrade someday, factory support, availability of replacement parts, especially wear parts(o-rings, tips, delivery hose, etc..) and price. You can get good equipment at good prices at the leading suppliers if you exclude a lot of the bells and whistles without resorting to buying cheap junk.