MDF is fine and it's easy enough to paint - ie find matt white works best as an undercoat. Two coats and then whatever colour you want... and go for 18mm rather than 15 as there's bugger all price difference.
If you want to stain it then go for 18mm ply, which is £28 at B&Q for an 8x4.
Size wise, i usually get an idea of what size i want to build and work out the wood cuts from each sheet. Then use that to finalise the size.
For example, if you wanted a 4x2x2 viv then you need 2 sheets of 8x4, but if you knock a could of inches of the viv you can get it out of one sheet. So if you decide to go for a 5x2x2 you might find that you can get a 5x2x3 out of the same amount of wood.. etc