Being lefty narrows your options somewhat as there are far fewer locks made for that side of the gun. The style of the lock "controls" the styling of the gun.
If you're in a rush, I'd agree to call Tip Curtis and see if he has anything LH. If you're lucky he'll have an "in the white" gun that is ready to shoot, but lacks any finish on the wood or metal. You get to pull it apart, do the finish work, and reassemble. OR not. Just depends on how "dirty" you want to get.
Buying a "random" second-hand ALR, I'd be real careful about buying "mishmash" of various parts that don't really belong on the same gun from a historical perspective.
Those are out there, and I'm sure that many of them make smoke, poke holes, and take game just fine, but they set a bad example for the sort of thing we do here. At your price point, this may be less of an issue.
There are no magic calibers in round ball shooting. Bigger carries further and requires more powder, smaller won't carry as far but takes less powder and lead. FIRST thing is to check your state regs, sometime they aren't logical. 54 has been super for me and Southern whitetails, where 180# and up is a big one. 45/50 are the middle of the road cals. 40 is light for deer (and prohibited in some states) but works for marksmen taking short-range shots. 40 and below are generally reserved for targets and small game.
Once you develop the most accurate hunting load, you can always try to find a lower-powered "target/plinking" load, same as any gun in my eye (handloader much longer that longrifler). That's a one-gun compromise. It's really best to work up the most accurate/powerful load for each gun and use the one with the best caliber match for the chore of the day.
I strongly suggest learning to load as Darryl and many others do: thick-wet, no-wipe loading. Don't fool around with flimsy patching. All that info is on here DOZENS of times. It's not the only way, but is worth giving a try.
If buying used, get a cheap borescope and/or run a tight patch/jag down it. You know about bores.
Great you've already got a connection out there. Get crackin!
PS, if the gun style you choose allows it, I suggest starting with a flintlock (and priming with main powder because it simplifies things). If you later decide you'd rather have percussion-then it's perfectly historically correct to convert that lock/gun to percussion. cheers!
PPS, if your big-game hunting is in heavy cover and ranges are short to moderate, you might consider a smoothie. But then I'd up the caliber to 62 or so, for better performance if used with shot.