Given appropriate gearing, Bromptons climb hills just fine. Indeed, that's the only time where they aren't frustratingly less efficient than most other bikes of equivalent weight (sure, they're a bit bendy, but that doesn't really slow you down).
If you can make the luggage work, then the only reason not to do it is that some of us would class riding long distances on a Brompton as Type 2 fun (OTOH, there are plenty of people who do all sorts of silly rides on them, and get on with them just fine).