Your experience with a Dually vs SRW hauling 5ver

Any newish 3/4 ton or 1 ton should be fine with that size fifth wheel. Buying used I wouldn't worry about the 250/350 distinction at all because it's either meaningless depending on options or it can be made meaningless with about $200 in aftermarket parts. Buy what you can get the best deal on that suits what you're looking for and go from there.