Just a few thoughts...
1. DA shooting, ie, just squeeze the trigger, is intuitive - first nature. Trained or not, high adrenaline situations are more reliable with a DA trigger.
2. A loaded revolver is more reliable, both for first shots and subsequent shots - including clearing 'duds' - just squeeze the trigger again.
3. S&W has put 'Internal Locks' in their revolvers for over ten years now and has yet to spend the first dime in court defending it's implementation. They are reliable. The majority of my revolvers have it - including my pocket protector and nightstand guns - their keys are still in their boxes.
And, finally, as a 'first' suggestion... find an S&W 4" Model 64 as a security guard trade-in. They run from $250-$340 depending on condition from J&G, etc. It is a SS version of the infamous original M&P, the Model 10. Many of the suggested 64's are configures as 'DA-only', ie, have a bobbed hammer and often no SA notch. No problem - DA shooting is intuitive, fast, and builds skills. Get some inexpensive .38 S&W Special plinking ammo - 130gr MC UMC from WallyWorld, 158gr LHPWC/LRN from GA Arms or Zero, etc, and take it to the range. Great for punching paper, pinging plates, or punching holes in cans. Pick up some +P-rated ammo for defensive duties - or 148gr Wadcutters (target loads), if you are recoil-shy. Save your extra money - add to it - later buy another revolver. You'll be better able to make a great choice then - and you'll already have a great home/car protector, too.
I lucked out several years ago. While perusing a local store's used guns, I found some security guard trade-ins - including this 4" 64-8 (The -8 is still the latest engineering release of the 64.). Never issued - ANIB - $309 OTD!
Or... take a leap of faith and get a S&W 4" 627 Pro - an 8-shooter .38/.357 Magnum that comes moonclip ready and with a few other goodies. Check S&W's site for more info/pix. It should run ~$800-$900 most places. Super revolver. Below it is shown with a 2 5/8" PC627 UDR ~ $60-$80 more. The 627 Pro has both an aftermarket grip and a HiViz sight.
Both can take moonclipped rounds or from a 5starfirearms.com machined aluminum speedloader/loading plate combo. Note my Remington R38S12 .38 +P 158gr LSWCHPs - my choice over .357 Magnums for protection. The greatest attribute of these revolvers is that they can take either .38's or .357M's - or, as I reload, wimpy 38-ish loads in .357 Magnum cases. YMMV.
Stainz