xyzzycoder:
All the advice so far is good. I would like to add, though, that your intended use might lend itself to the 'nitro-for-black' concept rather well. Black-powder proved doubles are considerably cheaper to buy than their nitro-express cousins, being considered generally unsuitable for hunting dangerous game, but are just as easy to load for, and much easier to shoot, than the big kickers.

A vintage Brit or European hammer double without lavish engraving should be obtainable for just a few grand. The Top Lever Hammer configuration is the most elegant IMHO, and even plain-finish guns often exhibit racy lines. If you can find one in .450, even better, as bullets are readily available in a variety of weights and configurations. This helps when trying to develop light-nitro loads. Buy Graeme Wright's book "Shooting the British Double Rifle", which has whole chapters on safely loading black-powder doubles with light smokeless loads and jacketed bullets.

This is only a suggestion of course, but its what I would be considering in your position. Good luck whichever way you choose to go.