Silvrettas are what you want, but skiing in mountaineering boots (downhill in particular) is hard, no matter what. I've found that wearing lace up ankle braces inside the boot (as tight as you can get them) stiffen things up considerably, making it easier to control the skis. Also, skiing with a sled is a whole new ball game, so if that is in your future, practice before the real deal.
you can put a regular alpine boot into the sivretta bindings. I have the older 300 style. The problem I have with mine is that there is a little bit of slop in the heel lock, so there ends up being a little bit of lateral play which would make the ski a little unresponsive. The 500's are prob a bit more solid, so you should be fine.