Chatting with @murphaph about this very thing recently.
Maximum grade is supposed to be around 1 in 50, ie a 10cm increase over 5m.
Two things that Murph pointed out were that the MM locos don't have traction tyres, so it won't take much of a gradient to slow or stop them, and that pushing trains up is even worse if they have standard tension-lock couplers, which ride up on top of one another.
Maybe post a rough plan of what you want with a few measurements, and somebody will have an idea.
Dave, the builder of the new Fry Model Railway is on here, but I forget his username. I'm sure somebody will remember.