For fitting suits in general, if you are "athletically built" i.e. your chest to waist ratio ("drop" in men's fashion parlance) is greater than 6", off the rack suits are usually a no-go. Made to measure is a good choice. Your tailor measures your body, you pick the suit cut & details like fabric, linings, belt loops, etc. and they email your specifications to a factory that makes the suit to your desires and ships it to your tailor. It is cheaper than a true bespoke suit but gives you much more control over what you want and requires the bare minimum of fitting and alteration upon receipt. Selecting a good tailor whom you can develop a solid relationship with is important. They can educate you regarding what is appropriate for appearance and fit. When it comes to belt loops, you may have to have them placed in an irregular pattern due to where you place your holster and accessories on your waist. If you wear an ankle holster for either gun, IFAK, or TQ, stay away from slacks that have a slim cut at the cuff.
Dress shirt fit can also be an issue if you are in decent shape. Be prepared to have them fitted as well. Depending on personal taste and clientele expectation, french cuffs (the kind that require cuff links) or barrel cuffs (the normal button kind) are different options as well. If you wear larger watches like a diver's watch you may also want the cuffs to be two different sizes. Also, your jacket sleeve length is important depending on the amount of cuff you want exposed. Shirts are also going to wear out faster from rubbing on your gun and other equipment.
Other things you need to consider is belt, shoe, and leather gear selection. Shoes that look good but are also comfortable and supportive are hard to come by. Generally speaking, lace-ups are better in these regards than loafers. Belts should be custom made and will be costly but are worth the price if done properly. Consider having your leather gear and belt made by the same person. Personally I would suggest having at least two if not three complete setups (belt, shoes, and leather gear), one in brown, one in black, and possibly one in cordovan. By going this route, you can wear the same suit multiple times but with a different tie and accessories to make it look like you are wearing a different outfit. It will also allow you to tailor your look to your clientele. For instance, older more traditional clients might expect "the help" to sport the more subdued look of a navy suit with black wingtip shoes and belt while younger more "hip" clients might appreciate the flair of the brown cap toed shoes and belt combo with a navy suit.
Ties (including tie tacks or bars), pocket squares, and watches also should be matched to client expectations. Generally speaking, try to mimic what the client wears as far as style unless the client looks like a sack of shit, because there are plenty of guys & gals with money that have zero fashion sense. A good rule of thumb is if there is any doubt, lean toward a more conservative traditional style of dress. Think more Brookes Brother and less Armani.