I partially disagree with Phillip, but its not easy to forge weld stainless steels. D12 and 420 or 440 will make a usable blade. If you worry about covering the cost of a real forge, as you stated in your other posts, then the stainless steel prices will kill you.
Start with the simple things and not attempt the high levels until you have some skills developed. I was taught start with mild, then do high carbon, then use low alloys, then higher alloys like D-12, then 200 or 300 series (bad for blades, but good for learning to forge weld stainless) then play with 400, after that go for others....
Remember in many cases, as the steels change so does the flux needed. Forget the "NO flux" welding here, and plain borax is not going to be enough for high alloys.