The AM21 should work just fine. However, rhambling is correct about the power issue.
I had my roof done last spring.
I actually screwed my dish to a piece of plywood and proped it up for a couple days.
This worked pretty well.
It's not too hard to aim on the ground.
A word of advice. Don't let the roofers take down the dish. I never did find all the bolts.
Mike