99% of the 1/8 buggy was done using free 2D CAD (Dolphin Partmaster) and 2.5D CAM to generate the GCode from the DXF drawings (Sheetcam). This is an exceptionally good combination in my opinion, total cost $150 for the Sheetcam licence. As stated above you'll need a machine control program, I use Mach3 because it is Windows based and I'm very happy with it, like most people. That's another $150 and that is all the software you need. If you want to do 3D work with your machine (see my exhaust manifold) you need 3D CAD and a 3D CAM program. Solidworks would be great for the CAD, as for CAM - there are a lot about, less for low cost though. I've used Meshcam with success in the past.
I have made a 1/5 buggy in the past, but I used 2 nitro engines for power. I have to admit I'm not a big fan of the chainsaw engines and their derivatives. If I did another big buggy it would be gasoline powered probably, but with a home made multi-cylinder engine... that's my ultimate goal.
1/5 scale:
3D exhaust manifold - drawn in Solidworks, tool paths generated with Meshcam, machined in 2 halves and welded together:
