I bought a CVA Accura V2 LR in .45 with a 30" 1 in 22" twist barrel this past November. I am very very impressed with it. It uses a 209 primer that goes into the removable breech plug, i.e. "in-line" with the barrel so it is in fact an in-line muzzle loader. You need to consider barrel twist rate if you want to shoot a muzzle loader long range, read up on that.
I decided to go with Blackhorn 209 black powder substitute and I am very very very impressed with that stuff. Very clean burning stuff, when I tested different charges shooting it for the first time I shot 8 3-shot groups and I tell you the 24th loading was just as easy as the second loading. No patching, brushing or any cleaning between shots. Back when I used Goex in my Hawkens percussion .50 cal I couldn't load a third shot with out spit patching or running a .50 cal brush down the barrel. As an aside, that old cheap Italian made TC Hawkens knock off was a tack driver. Every deer I shot with it gave up the ghost immediately. Of course I had extremely keen eye sight as a young man but never tried to shoot it too far, 100-120 yards was the farthest I took a deer with it.
Also decided to go "bullet to bore", i.e. where neither sabot nor skirted bullets are used. I'm using Arrowhead Rifles 300 gr XLD that I swage to the barrels bore size with an adjustable smooth sizing die in a Lee hand press. You do not have to go that far, the Powerbelt skirted bullets work well I hear as do sabots.
I'm not an authority but from what I've read 350 yards is pretty far to accurately shoot and humanely kill a whitetail with any muzzle loader but especially a .50 cal using black powder or a BP substitute. You may be able to stretch to 300 yards with a .45 cal but .50 cal is a lot of bullet to push that far. Now I am not saying it can not be done and I'm sure if you look around the web you will read of folks saying they took a bull elk at 350 yards with an iron sighted flint lock but one has to be on their game and know the ballistics especially with long range hunting. Bullets start dropping very quickly at that range and then there is wind drift to consider. If you are going to chrono your loads and work up a DOPE card then that is a different story.
If you are set on "at least 350 yards" (which implies more than 350 yards) then you might need to step up to a smokeless powder muzzle loader. Just to be clear these are NOT regular muzzle loaders but rather custom or semi-custom rifles that can take the pressure smokeless powder will develop. Now these are not cheap of course, they start at several thousand dollars.
Also to consider, if you want to use Blackhorn 209, start looking now. In my area at least it is very hard to find in stock. And 209 primers? You should start looking for them yesterday! They are extremely hard to find. the makers of Blackhorn recommend magnum 209 primers and, when I find some I'm going to buy some but I was fortunate enough to have Winchester and Federal standard 209 primers from when I used to reload shotshells. They make the gun go bang fine it seems. Just finished hunting the late deer season here in Delaware, had the Accura loaded for 11 days and on the end of my last day hunting (not having seen any 4 legged whitetails) I went to discharge the gun and it went off just fine but I am still looking for magnum 209s.
Anyway, long post I know and forgive me for rambling. If you want to long range hunt with a ML do a lot of research, read, read, read! Lots of good (and bad) info on the web from nice well intending people. Good luck