If you want it and it seems like a deal to you, buy it. Get yourself an RMC vent liner or drill the original vent liner out with a 1/16" bit and go shooting.
You mention that the builder is going to take it back and look at it. Is the builder willing to refund your money back or to pay for shipping? If the builder is not paying shipping (which could be up to a few hundred bucks roundtrip), I would check the inletting of the lock first and make sure...
Lyman is out of the black powder business and I don't know if Investarms is offering a fast twist barrel (I am only aware of their 1:48 as their fastest twist). You may want to contact some of the traditional muzzleloader suppliers and see if they have one. Also keep an eye on the gun auction...
The hunter model has a fast twist barrel and was designed for a sabot and/or heavy conical bullets so you will be fine shooting them. I have found that they also shoot PRB's accurately, but you will probably have to keep your powder charge in the 50-70gr range.
They are great rifles for the money. Have two of them in .54, one flintlock and one percussion(hunter model). Both of them are accurate with PRB, just have to keep the hunter model around 60-70grs 2f.