you'll find plenty of threads here on the board regarding the Prism, even more about the Inspiration. A couple about the Megalodon. Go through the threads, and if you feel like it contact the people who dive them with specific questions. I've done it a few times and so far everyone was very helpfull.
As always, your needs should determin the RB you get. For the Inspiration you'll find the largest aftermarket, so you'll be able to fit it to your needs if necessary. The Prism is the most compact of the three, yet alows larger tanks if used without the new cover. For the Meg you can also get three different scrubber sizes, in addition of using larger tanks. On the Inspiration they would either go off-board, or in a new frame.
In your price range should be all three of them. The Inspiration sells for about $7000, the Prism and Megalodon for about $7500. The only units I can think of that are too expensive for your budget are the CCR2000 and the elusive Infinito. Also used Cis-Lunars and MK15/16 are probably more.
The Inspiration is a pretty open book, as there are many out there and plenty of users and the press write about them.
The Prism was tested by the NAVY, and the results published. Like the CE-certification, I like the idea of having 3rd party evaluation data available.
For the Megalodon I haven't seen any such info yet.
Also consider availability of training, parts as well as the units themselves. Finally, if you have a chance, see if you can get RB-experiences on the units. All three should be available for that through their instructors, and it will get you a closer look and better feel for how they're set-up and work (for you).
But I'm sure Bill is right, the concensus will be Inspiration, they are quite popular.
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