First off, I am a newbie at this, so take everything I say with a grain of salt. I've done research and reading books, but have no real practical experience reloading yet.
I am planning on sitting to reload, but I have a pneumatic Craftsman workbench chair, so I went with the strongmount so that I could hang other things off the press (bullet tray, etc). I also went the route of mounting the press to some hardwood and then securing it to my workbench with clamps when I reload (so I can remove it if I need room for other activities) since my workbench is relatively small.
I thought about the 650 instead of the 550, and in the Dillon storefront, you can even get hands-on with all their machines before you buy. I figured that the 650 only added one more station (powder check), and the auto-progressive feature would mean more problems since I am new to reloading. That was my motivation for the 550.
I researched the dies, and ended up with Dillon steel dies. From what I read, the carbide dies will just last longer if you plan on commercial-level reloading. I don't, so I saved a bit and also figured that I might end up getting some Redding competition dies for 223 and 308 down the road anyway.
For each of my calibers, I bought the whole shabang to make changing to different calibers as fast and easy as possible. Mark referenced my earlier post with the items I bought for the caliber changes. All I need to do is swap the already-set-up tool head and the shell plate. Also may have to swap the primer assembly depending on the caliber - but that's fairly easy as well.
Even though it was more expensive, I am glad I went with the Dillon 550 right off the bat. I had it down to the 550 or the 1050 and went with the 550 due to cost and being less complicated.