I own an R51. I was an early adopter and bought a Gen 1, traded it for the 1911R1 and, when the Gen 2 came out, I bought one. Some of the R51's have some quirks. The two that stand out are a quirk with the magazines (may not affect you - shoot it first) and a quirk with the Leade/Freebore which will probably affect you if you hand load. I don't recall what the policy here on MT for recommending off-site forums but I'm on an R51 specific forum which you might find helpful. PM me and I'll link for you if you're interested.
You should also be aware that the R51 sights are set, from the factory, to Center Hold, not Target. I discuss it in my review of the gun here:
Review - Remington R51
How old? Tritium sights have a "best if used before" date.
Meh. If you can conceal it then it works.
My opinion on what gun you should carry? I have two and only two requirements for your carry gun.
1) Your gun must be reliable. It should go bang when you press the trigger, every time you press the trigger, and only when you press the trigger.
2) You must be reliable. You should only deploy the gun when you are legally justified in doing so, deliver bullets to what your intended target is, and only deliver bullets to your intended target.
I don't care what brand you prefer, what caliber you like, where on your body you carry, or how you carry.
I don't care about the brand of gun, as long as YOUR GUN is reliable, it can be a "cheap junk brand Saturday night special." I don't care what caliber you prefer, from .22LR & .25ACP, up through .460 Rowland because, statistically speaking, there's darn little difference between calibers when it comes to "making the bad guy stop attacking you" and, in fact, (depending on your preferred study) has between a 60% and 90% success rate without ever being fired.
Buy what you want. Carry what you want. As long as you're safe and your gun is safe, it doesn't matter.
Peace favor your sword,
Kirk