18mm Doors with 6-9mm panels will be fine. Just make sure the hinge you buy (if euro) is suited to that thickness. I forget which but one size requires a hole so deep the cutters guide comes out a little the otherside. If you have a bench drill you can grind most of the point off so this don't happen but in a drill free hand the bit may slip before it can pilot if grinded.
Sand the edges down to 240grit or more. 2 coats of primer sanded between with 240g then 3 coats of finish colour on top sanded between and you should get a flat smooth and mark free finish. I tend to roll the last 2 coats on after getting the brush into details and corners. After last coat use a finer grit.
EDIT: Biscuits work find in MDF, just made part of a bed using 15mm MDF and biscuits myself.