Ok, I'm glad you're prepared to deal with reality, many new guy aren't. I've been building these things for 37 years now, I'm not sure you could even buy the parts 37 years ago for $350-$400.
A straight barrel is $160
A caplock is around $150, flint locks go for at least $175
Set triggers are what?.....$65 or more?
Misc small parts for a poor boy , probably another $40 at least.
A decent piece of wood is at least $75
A barrel inlet and RR hole is going to go at least $75
Shipping all those parts around is going to go at least $80, probably more
Looks like parts will run about $700 more or less (likely more, not less)
This will get you a gun built that looks something like this if you know what you're doing....which you don't.
Then take into account you don't have any tools or a dedicated place to build one of these. Seems like a monumental task...of course I did it 37 years ago and have built 357 guns since then.
So, it CAN be done, but I'm not betting the farm on it.
In your price range I'd cast about for a Thompson Center, that's what I started on.
Not trying to pee on your corn flakes, but you said you wanted to deal with reality and there it is. SAve some money and get yourself a decent gun, you might get something used for $1200-$1500 that isn't too ugly.