I use the Wickes Trade series of water based paints. 2 coats - WT Super grip primer, 2 coats - WT Truewhite undercoat & 2 coats - WT Truewhite liquid gloss. Denibbing between coats with 180, 240 or 320 grit. I usually use a small 6" roller (not a foam roller, these leave loads of bubbles behind) to apply all the products but keep a decent 2" brush to hand to deal with any tight corners or awkward edges.
I've been using this system for a few years on MDF and find it applies well, builds up nicely, stays white (no yellowing at all) and best of all you can clean up with water.
I usually put the primer coats on pretty heavy at the cut edges of the MDF and then sand it back level with a block and 180 grit paper. In warm conditions the gloss sometimes benefits from a bit of Floetrol paint additive to help it level out and keeping the wet edge live.