enamel is very durable and about half the amount per gallon than single stage urethane, the one down side is that unless you're a highly experienced painter it doesn't lay out as easily as urethane or lacquer for that matter.
in the long run it's a track car, and i mean nothing bad by it loke you said it'll see some damage, and it'll see wear and damage that most cars won't so i would go with an enamel for track purposes, but for looks i'd go urethane. it's durable but not as much as enamel but urethane is much easier to get the gloss and layout and still keep quite a bit of durability. and even with a little bit of orange peel you can buff out urethane much easier than enamel.
painting enamel over a primer is fine, as long as you double check to make sure the primer you are using is compatible with the enamel paint line you are using...i always recommend using a primer/sealer surfacer to help hide blemishes in body work.
i had to come back and edit how much you'd need. if you pick enamel you should be able to cover your whole Z with two coats no sweat so a half gallon maybe 2.5 qts. If urethane, i painted a subaru justy today, about the same surface area as a Z, i used about 3 qts. but it was a taxi yellow and a color that doesnt cover very well. a very close barrier coat like a non catalyst sealer helps those hard to cover colors cover better so you're not putting on 6 coats to get it to even all out in case you burned through your primer anywhere or have fiberglass, bondo, metal, previous paint etc spots showing anywhere.
I've sprayed Starfire Enamel and had great turn outs with great shine. a gallon sprayable is 71 bucks so it's quite unbeatable. you can check out trinity 1945 products and such and other colors at the link below, the link is for a silver gun metal enamel.
http://www.paintforcars.com/kits_gunmetal.html
good luck with your track car, i'd love to see pictures when you're done painting it which ever way you go
cheers