I'd pick what is affordable out of the Adaptec performance line. They should all be Windows compatible. When I set up a server I like to go one of two routes. With a a nice budget, RAID5 with 4-5 disks. On a tighter budget we sometimes go with RAID1 and just 2 disks with a 3rd as backup...depends on the server config too as to what your options are. The 1U Dell's I like are obviously less flexible when it comes to multiple drives.
But in either case, I typically have the OS and data on the same server, with a partition large enough for the OS, software and future updates, etc. You could certainly run the OS on a dedicated drive, SSD or otherwise, but unless you do a separate RAID there, you're opening up yourself to more risk IMO.
Of course all of this gets mitigated somewhat with redundancy for the servers and some sort of NAS. But I don't know enough about your environment.