Unless you can find a used copy, you can't get 5.0 (or 6.0) any more. They quit making 5.0 a couple of years ago, and 6.0 had so many bugs that they replaced it with 6.1 about 6 -8 months ago. I've used every version of CF since 4.0, and so far MX 6.1 has impressed me the most with it's speed and performance. Not to say the others were bad, I've just noticed a few little perks with the new one. Plus, with MX 6.1, you have several new tags and capabilities that are not available in earlier versions. Not to mention that with the new version, you can get support, upgrade options, etc...
As far as Blue Dragon is concerned, while it may be free there are several features of CFMX 6.1 that it does not support, unless they have just recently put out a new version. So, there is the possibility that you can write your code for MX 6.1 and it may not work on Blue Dragon.
Hope This Helps!
Ecobb
"My work is a game, a very serious game." - M.C. Escher