3000gt sl dying problem?

My friend justin and I are in the market of looking for an automatic 300zx or 3000gt for him (no flaming with the automatic... It's what he wants.) Anyways, We went and looked at this 3000gt sl (I'm sure it's an sl) at a dealership near the highschool. The dealership guy would'nt let us drive it since we arnet 18 but i asked him if he would start it up for us. He did and it would start but he had to keep giving it gas to keep it from dying. After like 30 seconds or a minute, it could run on it's own and it sounded good and quiet. I went to back looking for smoke coming out of the exhaust and i didn't see any (I forgot to look when he started it), it sounded all good and we let it idle for like 5 minutes and didn't seem to have any problems once it got going. It's 3900 dollars and it has a really shitty yellow paintjob. We want to know what the problem is with it wanting to die. I thought it could be a clogged fuel filter or something having to do with the sparkplugs. I wasn't sure. Anyways if we could get this thing running right for cheap then he wants to buy it and with his left over money re-paint it. The interior looks good and everything is good except for the cold dying and the paint. We still need to test drive it to check the brakes and all that. Can anyone help me with the start up problem? Thanks!