Three hours at 75 -80 MPH ?
In the scooter world - I think I would trust only a $12,000 Burgman 650 to survive those kind of repeated runs in decent health.
But, before I'd drop that kind of money on any scooter - I'd be looking at motorcycles. Where you can get more cc's for a lot less $ - and not have the late-life death rattle of those big Burgmans. When that massively complicated CVT goes - most folks scrap the scooter for parts.
If you don't expect to ride anything for 50,000+ miles - Yeah, you could take that $12K and look at the 650/Burg and/or gently used bikes of all kinds & sizes.
A 500 Kymco - even if it does not develop the refusal to start issue - ......at 75 - 80 MPH for 3 hours, repeatedly?!
I can't see it staying healthy for very long.
If you could knock 10MPH off of those runs - the Burgman 400 and maybe even the Kymco 300's might suffice.
If you could do mostly secondary highways at 55 - 65 mph any 300 - 400 scooter would keep you quite happy.
If you have been off of two wheels for some time - you might want to start small and see if it is really for you, again.
Guys are really happy with their Kymco 300's and the Burgman 400's.
Stig