It has to be a coach that you can live with first. But from pure mechanical side of the equation, the 05 Dynasty chassis with tag IMO will provide you a longer term foundation. Making mods, and refreshing a coach at this level, will provide you and opportunity to make this into 'your coach'.
Not knocking the Allegro, but it is not in the same league. But, opinions will vary...
By the way, the 42' is a nice size, still under the 45' level that requires extra drivers licenses in some states, and that 2' over a 40', gets back a full bay and usually a larger set of Grey/Black total capacity. The one possible downside to the Dynasty is if it is the ISL, it could be on the lower side of the power to weight ratio then if the ISL is in the Allegro. Even so, I'd just enjoy myself and go a bit slower up those hills in the Dynasty...
Last factor is important. You know the owner, you know it was garage kept, and you should have all documentation available for you - that's real nice to have.
Best of luck,
Roo II is our 04 Country Coach Allure 40'
OnDRoad for The JRNY! Enjoy life...