I bought one a month ago, and really like it.
To answer your questions:
- 45ACP DA/SA - yes, though the DA pull on mine is terrible. Feels like a toy cap gun. SA is fine.
- comfortable in appendix carry, I find it faster to draw in this position and I do A LOT of sitting in traffic, in urban areas. Should be fine. I'd use a shoulder holster instead, as I'd think you'd have the lap belt in the way?
- Relatively easy to find holsters and magazines. Oh yes. Mags were 30 bucks on sale at Midway a few weeks ago.
- reliable, I know its HK but I have to ask. Non-beliver - off to the HK factory for you. Work Makes You Free!
One downside is that most lights won't fit it, due to the silly mitten sized trigger guard that allows for limited rail length. The Crimson Trace 204 fits fine.
For your set up, that's what I'd go with - get a big shoulder holster than can carry it, CT 204 light/green laser combo for 275 bucks. Set it up for both laser and light. Any issues - the goon has a face full of 100 lums, a green laser, and a .45 hole staring back at him. I think he'll suddenly remember that he has other pressing issues.