The size of gun that you can conceal depends on how dedicated you are to carrying. For example, I know people who think a j frame is too much to carry. They'd never even try to carry a USPc. On the other hand, I know a lot of people who carry guns in the Glock 19/Sig228 or 229/P2000/USPc size range without any trouble at all. Hell today I carried a Glock 19 and a Sig 228 (at the same time) with a polo shirt and no jacket. It just depends on if you're willing to put in the effort.
You shouldn't have any problems carrying the HK USPc. The most important thing I've found to concealing is getting a good holster and a good belt. If you skimp here you'll have a difficult time hiding it and you'll be pretty uncomfortable while you're at it. I'd expect to pay around $100 for a holster and $50-100 for a belt depending on what you get.
If you're looking for deep concealment, I recommend belly bands or Smart Carry. These will leave no belt loops or clips for others to see if you have to tuck your shirt, but they probably won't be your every day holster set up as they are typically more difficult to draw from and aren't always as comfortable as a normal holster.
I've become very fond of appendix IWB (for me it's more like 12 o'clock instead of 1 or 2), and I think it is the best option from a concealment, in fight access, and retention. However not all body types work well for AIWB so you'll need to do a bit of experimentation. 4 o'clock (behind the hip) is a good second position especially when you can use a jacket to help concealment. You can usually hide larger guns here (as opposed to AIWB). One downside is that bending over will usually cause you to print.
For mags I usually carry two in my back pocket. Probably not the best way to go but it works for me. A lot of companies make mag carriers, but most tend to be outside the waistband which I don't feel conceals as well.
And finally, congrats on getting into the carrying world. Once you get the gun, find a good shooting class and learn to fight with your gun (not just something like a NRA pistol class, though that is a good place to start if you're new to shooting). Carrying the gun is important, but so is being able to use it effectively.