Well, considering I just went through a full day of this rich dickhead and his buddy giving me crap about my Canik
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First, I would choose 9x19mm since the gun is for training and it will lower your costs by 33% from 45 ACP.
For this role, I recommend the HK P30 Lite LEM. HK has the reputation that you're looking for without stepping into a $1,500-$2,500 1911. It is a very well thought out design. It will work well through a very high round count. Just add Trijicon night sights, a few extra mags, and you're good to go. The HK USP is also a good choice, but I really like the P30. The VP9 is a respectable choice if you need a striker fired gun. I prefer the hammer on the P30 Lite LEM for control during holstering while carrying in the appendix position.
Start reading an insane test of the P30 here:
http://pistol-training.com/archives/1406
If you want to get really crazy, get a SIG P210 (standard model). These guns are highly respected and almost everyone will want to shoot it. I do not know how reliable the new P210's are, so it may be a problem like a 1911. This gun is for instructional purposes, so reliability is important...which takes us back to the HK P30.
Also, going with a standard Glock 17 or 19 is a statement. It's a no frills working man's gun. This contrasts with the "Tacticool Rich Boy" HK P30 (that only cost $150-200 more). You can do all the work yourself in case of breakage, swapping sights, etc. Like the HK, it's fine out of the box. Light mods like a KKM barrel, Trijicon night sights, and a modified trigger with 3.5# connector with NY-1 trigger spring are all easily completed at home. The gun will work for a very high round count and parts are cheap.
A 9mm 1911 such as the STI/Staccato P would do well for you so long as reliability was not compromised. People seem to like them and would likely want to shoot it.
You could get a customized OZ-9 from ZEV, but it seems a bit pointless. It'll make a "statement" to some of your clients. Frankly, it's a bit over the top like a custom 1911.
Personally, for an instructor's gun, it's a toss-up between a Glock 17 Gen 5 MOS and an HK P30 LIte LEM. I really like the HK, but the optic mounting ability of the MOS guns means you can teach a red dot class (or demo one).
