VMWare's Fusion 5 has a system requirement of Mac OS X 10.6.8, so you do not have to update any further. The do recommend that you use 10.7 or later, however. You can get Mountain Lion from the Mac app store, or if you want to stop at Lion you can call Apple's sales department (800-MY-APPLE in the USA) to get a download code for it.)
You may also want to consider Parallels, a product similar to Fusion. It has similar system requirements.
You would also need to purchase a supported version of Windows and install that as well, to run under Fusion.
Best of luck.