A 2K primer will be a good high build (to fill sanding scratches etc) just make sure you find out about how whatever you're using works with metal. Many primers today are DTM or direct to metal and have a little additive so they can be sprayed onto bare metal. Some however do not, and will require the etch primer (additive in the DTM) to be sprayed on the bare metal only before you apply the primer.
As far as a sealer, IMO it depends on alot of things. And there's alot of different mixes. I personally use a wet-on-wet sealer that is mixed from our regular primer and tinted with single stage urethane.
If the car looks like this: (alot of different applications of primer, different colours then desired etc)
I do a sealer. It does a number of things: 1) It allows you to put less paint on for coverage ($$$) and in a case where the primer isn't tinted (like above), this will help alot.
2)If you're doing a metallic colour, it will give a nice smooth base for the colour to lay on. Vs. spraying directly over the sand scratches (even though they're fine) The sealer will fill MUCH more then a basecoat ever will (in the neighbourhood of .8-.9 mils per coat vs. .2 mils) to allow that smoothing to happen.
As for the polyester basecoat. A pretty basic rule of thumb is if its a urethane clear, its likely polyester base. If its an acrylic clear, I'd stay away.