The opening you would need for our loft hatch is 726x562mm this gives a clear opening of 700x530mm, also bere in mind any loft balustrade you may need around the loft hatch. http://www.northernstairparts.co.uk/Plastic-Loft-Hatch-Model-P5370L/1600.htm
I would go primed MDF, surely you only need two top coats of Acrylic or gloss? With the mitres I always use a plane, like a block plane or smoothing plane to par off either the front or back, whichever is touching in the wrong places, since you are painting it wont matter which way you do it as...
They look pretty good to me... sometimes people buy things that just dont fit or they change their mind then that leaves you to grab a bargain off them, if in doubt ask for more pictures, descriptions and make sure it is solid oak!
The minimum head height over the stairs is 2000mm, you need to keep this if you ever want to sell the house again, creating the cupboard is the best idea and simple to do, builders will use that wall to fix a winder staircase to when converting your loft so bare that in mind should you be...