primed door - what top coat?

Personally I would go for water based although water based doesn't give as much shine as oil based it will stay whiter and shinier for longer and also the added bonus of not having the smell of oil based paint.

Depends on how much time or money you're willing to spend really. Pre finished doors are fine as long as the fitting is spot on as there isn't as much room for error. Once they're hung, the edges are likely to have been planed so will need Priming, undercoating, top coating maybe even knotting before hand. If the hanging of the doors is good then a lot less work than painting each and every door yourself. Just make sure you get a finish that matches the finish of the doors to paint the edges in. Sometimes you can get a touch up paint that matches the doors. Pre primed doors aren't white. They'll still require at least 3 coats to bring them up to a nicely glossed/satin finish. They'll still require 2 undercoats and a top coat in oil base or 1 undercoat and 2 top coats in waterbased. Tikkurila helmi 30, little greene are good systems for water based. So nice to use, solid hard wearing finish. Msp is also a good option as you won't require a separate undercoat and top coat. Just 3 coats of the msp in your preferred finish. Also a nice hard wearing finish.

Hi, On oil base prime you should used oil top coat, On water base prime you need to used water base top coat, Greg

Oil pased paint for a front door with oil based undercoat, one undercoat with two top coats

Please don't use oil based, there is no benefit these days and they cone with a health risk due to toxic fumes. Have a look at Zinsser Allcoat exterior (make sure you get the water based version). It's designed to be used as an exterior paint but is also suitable for internal use. You don't need seperate primer or undercoat and it dries very quickly and stays white. So just gove it two or three coats of that.