some of the primers can also be used as a sealer, you will have to check the manufactures specs on that one. As far as single stage or base/clear, I would go base clear, if you screw up it is easier to fix. With single spray after you are done spraying if you have any runs you pretty much need to respray the whole panel. I am in the same boat as you right now except for I already have my car primed. In the next couple of months I will be ordering the paint and spraying it. I looked at omni and the price is good however I will probably go with the shop line (basically the same thing as omni but from a pro dealer).
I have a dart and to paint the doors, body, trunk and fender I figure its going to take about a gallon of paint and clear. really depends on the color and the brand as to how much you need.