There is a VM tool that calculates capacity and will tell you how what server resources you should buy. If you can't use the tool, then you can estimate.
I would add up the total peak in-use memory of all the servers you want to virtualize. Suppose that all 3 servers have 4GB of RAM installed, but total peak mong them is only 9GB. Since you want to add more VMs in the future, you want to spec out those as well and have at least that amount of RAM available. For arguments' sake, let's say that SQL will need 6GB, SP 4GB, and LYNC 4GB. Given 23GB of RAM, I would aim for 32 as a minimum, but if you can afford it, go for as much as you can afford. Sixty-four or more might sound like overkill, but trust me, RAM gets eaten up fast.
As for CPU, you can count the total cores in use today and have a look at peak utilization over a 24 hour or 1 week timeframe. Add up the total cores and total the CPU utilization. This should give you a rough starting point. Suppose each 2850 has 2 CPUs and 2 cores (total of 4 cores per box). If utilization is 30% or less, you can effectively cut the CPU requirement in half and run on 2vCPUs (2 cores). If CPU is less than 15-20%, I would try to run those servers on only 1vCPU. The less the better efficient use of your hardware. For the future VMs, you might need 1, 2 or 4 cores, depending on the size, user load, etc. Total core count might range from 7 or 8 to as high as 20. I would spring for the highest core count CPU with the largest cache size you can afford. Gigahertz and power and less important, although the lower wattage CPUs are probably more economical...Typical vCPU to pCPU ratios to strive for are from 2-5.
It sounds to me that you would be fine getting a six-to-ten-core CPU and 64GB of RAM and should be fine. I would buy the largest DIMM RAM sticks you can afford and don't fill all the slots. If you need more RAM, you can add it later without throwing out the existing RAM. You can also add a 2nd CPU if needed. Hope this helps.
