Totally agree with Mike A but I wouldn't rule out MDF. It's cheap, nice to paint and easy to join (permanentely) if you use a biscuit jointer. If you're using screws with MDF you'll most probably have to use battens. Joining MDF corner to corner with screws is possible but you'd have to have perfect pilot holes and countersink everything and have the right screws. Even then the joint will be weak simply because of the material itself. A slight twist and the screws will be useless.
I would only ever biscuit joint MDF and if you have plently clamps and good glue you can get really good, strong results. Just as strong as a ply built machine but thats only because its glued and permanent. Cam fixings, screws, nails, bluetac, anything like that and you'd be better with ply. Especially if you're building something you want to take apart to move it. Melamine is another option but the inside is chipboad and not as strong as ply. I use whatever I can get for free but for a beginner, either get a biscuter and learn how to use it or just use ply. The only two advantages to MDF are, its usually cheaper (might not matter to you) and its easier to paint the face (no wood grain).
Personally I don't really like battens. I know a lot of original cabs use battens but that was the quickest / cheapest method. There are other ways to fix a cabinet together. A lot of the flat pack arcade type things use cam locks which is probably cheaper nowadays.
Consider using a 'key' on each side panel to help strengthern CP and CP front plate. Similar to battening but easier to do if you have loads of small battening bits. Look at some old nintendo cab plans and you'll see what I mean.
Regarding the CP hinge. You might want to consider having it fully removable as you may want to have different CPs or work on the CP away from the machine.