If I understand you correctly you want to take some older machines and use them as VM hosts, and move the guest OS's to newer hardware as it's available?
Okay...in broad terms, yes. That's very possible.
Now, for your requirements, we can't help you there because we don't know what exactly you're running. How much they'll need in disk space, how much they need in I/O, etc.
If you're running a host with crappy disk I/O, your VM's can choke. If you're running a lot of light tasks, and they manage to keep mostly in active memory, you may not have any trouble (or at least have acceptable performance).
If you're looking to do something on the cheap, we've been running VMWare ESXi on some Dell 2950's with ~13 VM's and not had much trouble; mostly light duty stuff. We are working on getting a second server to copy images to as a backup in case there's catastrophic failure, since losing the host will kill all the servers. For our situation, the time it takes to bring those servers up (and the occasional longer term outage as we copy VM images to another place, as the transfer is painfully slow) won't kill us. Of course they have RAID for availability, but that's not really a backup of any sort.
The only way to know for sure if this will work for you is to actually install a hypervisor like EXSi (free) or hyper-v and start creating your VM's and services and migrate them, and test them under actual load. My gut feeling is that you'll want a better server with 16+ gig of RAM (more if possible) and at least 200+ gig of hard disk space if you're using a light server. You also need a server that supports the virtualization features. And get a spare server to act as a backup.
In full-on use you'd get a SAN of some kind to act as your file storage, and two servers to do the actual hosting, and then you can purchase functionality that allows dynamic balancing of your VM's between the two hosts. This adds some redundancy and flexibility in rolling out your VM infrastructure.
Summary; you're on the right track, but you're giving estimates that are probably on the low side for performance, and you'll only know if they'll work by trying it under your business's real-world load and use.