Pac-nor will probably do it. However, all my custom xp-100s have Lilja barrels (I've used shilens as well). You can get a 15" Lilja SS barrel for about $160 and they are top notch. Also, put out a post in GUNSMITHING that says you are looking for a good gunsmith in your area. Finding a GOOD local smith is a great asset. Anyway, good luck. BTW, the Nikon 2.5-8x handgun scope is top notch...
No problem on the 22-250. If you are looking for a fast 25 caliber for an xp-100, a 250-3000, a 250-3000 AI, a 25 Souper (aka 25-308), 25 Souper AI, 257 Roberts, 257 Roberts AI, and a 25-284 could all be made on your action without modications, except, obviouosly, for a new barrel.
I'd consider a muzzlebrake on the 257 AI or the 25-284...
7mm BR is a standard 473 case head, so just about any standard SA round would work without modifications. (I assume you have an xp-100.) The bolt face could be opened to accommodate a mag case, but the xp-100 action is too short for anything longer than, say, a 284 win case.