unless you're going from calipers to discs and your aluminum bike is within the last 10 years or so, it's not that much a PITA. *do* pay attention to the bottom bracket spec. Either look for the same, or be prepared to buy a new BB/Crankset to accommodate. Other than that, it's very doable (and since you've upgraded stuff already, I would guess you kinda know what you're doing and have the tools)
as far as what frame for $1500? I got nuthin. My last build was a $600 steel frame
