As you've stated, you're already in the hunt for a custom. What are the advantages? Well, you get a hand made gun, that shoots like it's supposed to, that you can enjoy to whatever level of HC/PC you prefer. You can use your imagination and create something that is not particularly HC/PC and enjoy it just as much. I've had a half dozen or so TC sidelocks, and they're ol for hunting, and just enjoying shooting, not a thing wrong with that. But, I also have three customs, all flinters, and they're the cat's pajamas. Beautiful, and accurate. There's just something about a hand made gun, that conjures up thoughts of years gone by, and adventures long past. I was hooked the first time I shot a flint longrifle, not all that long ago. Don't be discouraged by the differing opinions. If you can afford one, then get some information, find out what fits you, and ask a bunch of questions.
I would highly recommend you give TVM a call and talk to Toni and she could give you more information than you could find in books, or on the net in years. That will go a long way to help you decide what you want and where to get it. A .45 is a fantastic place to start, all the makins' are easily found, and you won't need to cast ball right away, unless you just want to. You can shoot and enjoy any muzzleloader, factory or custom, but most of us here wanted to take the step into a truly hand built gun, and it changes the whole experience when you do. If you like muzzleloaders, and it appears you do, then you'll never be sorry. The only big thing I think you should keep in mind is fit, make sure the gun you buy fits you. I have one .36 flinter that is 14-1/2" LOP, with a 42" barrel, which is just too big a gun for me to shoot comfortably. I keep thinking I'll get rid of it, but never do, I just love the gun. It's up to you about worrying about PC/HC, some do, some don't, just have fun.