I would disagree with you in that MDF has to perfectly sealed and if not will swell to double if it gets any moisture. Plywood is much more stable especially if the correct type is selected amd if you went that extra bit to seal. Plywood is the classic as it lasts, MDF can not be fastened that well relying heavily on glued joints and if it takes some knocks will delaminate. MDF machines nicer and easier To machine and smoother to prep but that's about it.
As for brims The last i bought off them was 8 years ago so maybe they closed doors but the name did come up in a search based in Brisbane. Mister Plywood is another good supplier in nsw and qld and they have the classic 3/4" ply thickness? Ensure you go for a laser ply or similar as then the sheets will be perfectly flat, and no knots and nice to machine. Cheap construction ply of internal ply will be only have good layers on the outer surfaces and not in the centre.
http://www.misterplywood.com.au/Hope that helps
