I'm not sure running a CAD program in a virtual machine (VM) is a terribly good idea. You take a performance hit because you're running it virtually and the graphics "card" in the VM is almost certainly considerably less capable than your real one. Think 3D work. These are just general observations on using VMs and not specific to Parallels or Mac.
As you mention the more RAM the merrier because whatever amount of RAM is used by the VM is no longer available to the host system. How much RAM does your Windows PC have? Add a couple of GB for OS X to keep running comfortably and there's your answer.
If you want to buy a Mac by all means do but wanting to run all that Windows software says to me you really should be doing it on a Windows system. VMs are great for occasional light use but not so good for regular heavy lifting.