I've seen underlever guns shot at Vintager events. The only requirement is that the gun have the two barrels aligned SxS as they are supposed to be.

None of my 3 SxS's are in any way special and I've never had a disparaging remark made about mine.

Buy what you can afford and you'll have fun shooting Vintager events. But I'll give the same advice that I always do about buying guns - if a little more money will get you a lot more gun don't skimp.