Go to somewhere like Halfords etc.
Buy "Davids Isopon P38 Easy Sand" if you cant get easy sand, get the regular one.
Its an polyester based car body filler.
Dont get teh one with fibre glass strands in it.
My father has been using it for years and years in proffesional decorating (Kensington, Buckingham Palace etc)...
Reason being, it doesnt shrink, doesnt crack liek plaster...sets and dries quickly, meaning you can get teh job done much quicker.
You can reconstruct shapes with it...repair window ledges..anything.
Can be drilled and tapped even...hard as rock this stuff.
Mix it up, not too pink (teh hardener is red, teh more hardener teh quicker it sets and teh pinker it is), put it where you want it using teh spreader provided.
I would paint the plaster board first to seal it...something liek an alkaline resistant primer...then fill it, as it will mean less damage to teh plasterboard surface when you sand it down.
No metal edges, will look perfect if you take teh time.
All you do, is add more until teh surface is flat and uniform. Just on teh joins, rough edges etc.
Trust me.
Something about this size, then yopu have some left over for the next job.
http://www.edirectory.co.uk/pf/880/m...re/pid/1346862
Your right about MDF not taking paint up in teh same way as well...will look very smooth compared to plaster board walls.
Also...that much MDF in a decent thickness, will be pretty expensive!
I would also allow teh whole thing to settle, once your ready to start filling...after the board is up, so it can aclimatise to teh room its in.