The Minolta lens are actually pretty good. I grew up with photos taken by a Minolta
Since you have a film slr, you should have experience using lens with IS. Personally, I don't find it that useful since camera shake is only 1 part of the picture. If your subject is moving, IS will not help you get rid of the blur. I prefer using wide aperture lens and faster shutter speed even though I have to manual focus.
However, since you have already invested so much time into getting to know the canon system and you obviously like it from your previous experience using it, go with the 550D. Personally I don't think full frame will take over. Even if full frame drops to a $1000 price point, there will be people around who want to buy $500 dslrs. Of course when we make statements like that, we are only guessing. I don't think anyone has inside information at this point of time. If you are worried, start with full frame lens. They will perform better anyway due to the 'sweet spot' effect.