Water based paint works a treat on MDF, it certainly won't cause the MDF swell. I have worked with MDF quite a few times before on other projects and water based paint works real well.
A sample pot is the way to go if your doing small areas. I bought a 1 litre tin and only used 3/4 of it on the whole project.
I didn't use a primer, everyone said to, but the surface of MDF is actually quite dense and didn't such up the paint much at all, that said, I did put on 2 coats and each was quite thick as I used a roller. If you needed to paint open cut ends, then a primer may be a good idea. In my project the t-moulding covered all the end cuts.