Maybe cloud hosting is something for you. I'm using iwStack.com and there you can create so many servers you like, all to a very good price.
When somebody don't need it, or take the weekend off, you can turn off servers, and only pay for disk and IP.
You can start small and go up to 16 GB servers with 12vore. You pay per hour, so if one co-worker need a big VPS for some hours, a day or so, you only pay for actually use.
With iwStack you are in control, you can create, run, stop, and destroy so many VPS servers you like.
Check out iwstack.com and see how much money you will save on going to the cloud.
Well there are some good things and bad things about putting all your eggs into one server. If something devastating happens to that one server, everyone will be down until you can bring it back online.
On the plus side, you can save money and have one very nice server with lots of redundancy such as Raid-10, redundant power feeds and power supplies, hot swap drive bays, etc...
Its hard to recommend a server to you though without knowing the specs of your current 8 VPS. Are they all using a lot of space? If not, then go for SSD drives on your server as you will see a great performance boost using these versus slower rotational drives.
Also, what is it each of these VPS's hosts and how much traffic/load do they usually consume?
Really what you should think about doing is instead of buying the Dedicated Server out right, think about possibly renting a Dedicated Server and have it Managed by the Hosting Company so in case any hardware issues arise the company has the obligation to get it repaired. Technically for that amount of money a month it should be no problem, it just boils down to hardware configuration.
Though if it was me I'd run the Dedicated Server off of ESXi or a Basic Linux Install and then each VPS. Though if you want to run it all off of Windows be my guest, it's your setup. Just remember you're going to have to either rent or buy your own licenses for Windows on each VPS including the Host Server as well. Once you factor that in, it might be more cost effective to go with something I suggested.
You should tell us your resource usage for each VPS to help you in deciding what to use. It really all depends on that.
However, if I make a guess based on the workload you described (web based tasks, such as downloading and uploading) I would say that you would be covered with a low to medium VPS for each coworker. A VPS with 2GB of RAM should be ok. If it will be based on SSD storage it would be preferable but not 100% necessary.
Something to note if you go the dedicated server way and split it up: Windows licensing. You must license the VPSs you will create. There are two kinds of licenses, Standard and Datacenter. With Standard you get limited virtualization rights. With Datacenter you install as many as you need (as long as your server's hardware can sustain it). Note that Datacenter costs 4-6 times more than Standard and you can also get multiple Standard licenses. The sweet spot depends on your provider's pricing, ask them to get pricing on both. It might be more cost efficient to use multiple standard licenses, although with 8 VMs I believe you’d be better off with Datacenter. Obviously, if you get VPS hosting or cloud instead of a dedicated server, the Windows licensing costs are included in the monthly price.
And something else, you can use Windows Server 2012 as the base OS and virtualize with Hyper-V. You can also use Linux for the host. In either case, you still have to license the VPSs. And since you’ll be paying for it and Hyper-V is rock solid and has pretty much every feature that you might need for virtualization, you should use it since it has a familiar interface you can work with.
And finally, if backups are important to you, make sure you have a backup strategy for your dedicated server. If you choose a VPS or cloud provider this is taken care of for you.