I think you ask the wrong question. First, the question is what people do. You want enough RAM to keep everything on disc that works, and enough CPU and IO to work with everything that is needed.
Here we go:
- A handfull off user on a dev environment can be very taxing. OTOH they may not. I use a AMD Phenom II recently as build server / TFS server / dev database server, and while you may smile at the performance, let me tell you that during the day, 90% of the time, 5 of the 6 cores are parked.
What I do see is problems on the IO Side that I am happy with. I have a 4 disc RAID 10 for the OS and virtualization, and during builds the build server trashes them. Bad.
Now database, this will very likely not tax your CPU but be totally IO bound. You have no chance to keep 100gb in memory (on a decent budget without making the server sql only) and even then it may simply not matter because transactions go to disc. We can not say what you need here without knowing WHAT THE DATBASE DOES. 100gb sounds like a lot, but it may be dead data (texts, images, version control) or active data (financial tick time series that gets aggregated and scrubbed). I have a 800gb of the later and I run that on about 10 fast discs AND IT USES THEM UP - all while using half a core of my server. Ouch.
In general, I would gl for a server with enough RAM to really handle all your stuff, and that means you likely want (unless inactive) 16-24gb for the sql server instance ONLY. How many VM's? Depending on what and the database patterns anywhere from 8 to 32 discs may be appropriate.
I would really look at the CPU last here, unless some of the usages is calculating ray tracing stuff or video encoding on the CPU.
Depending on the active set you could go with a end user based system (at least AMD goes up to 16gb RCC on a micro atx board - just bought one, plugged in an Adaptec raid controller and a SAS cage for 8 x 2.5" discs) or need a professional system capable of handling 64+gb RAM. Really depends. TFS, BUild server and lab management + "throw away vm's for develoeprs" may really tax the system.
But, again, your CPU wont be the problem. RAM will, and discs. Discs really will. Spend a lot on discs.