Rebuilt Motor Leaking From Head - Why?

1) My exhaust manifold has a 3 inch crack running down the middle. I'm broke, how badly do I need to worry about replacing the headers before I put the motor back together?

2) The motor in the DOHC I just bought has been recently rebuilt. Or I should say, the DOHC that was originally in the car was replaced with another rebuilt DOHC. The top-end is in great condition but the head is already leaking at the #4 piston at the back of the head, covering the bell hosing and firewall with oil.

I've got the head off and everything looks great, the mating surfaces are to spec, valves are all good, and so on. The old MLS gasket looked worn out but that still doesn't explain a head gasket failure on a '99 DOHC MLS gasketed motor. So it could be improper installation of the old gasket, or the rebuilder didn't use new head bolts, or... what?

There was a ton of oil in the coolant resevoir so this had been happening for some time. The car also overheats sometimes... Most of all I don't want to put the motor back together until I know if I need to replace the header and / or inspect the motor's bottom end.

Thanks.

Sounds like the head gasket is bad. You must use new bolts anytime you remove/replace the head, always, or the head won't stay sealed.
If you have a cracked header, you run the risk of cold air getting sucked up during exhaust reversion. If the cold air hits the red hot exhaust valves it will crack them and you will be replacing or rebuilding the head.

So it could be improper installation of the old gasket, or the rebuilder didn't use new head boltsThat would be my guess...improper installation. New bolts and correct torque is everything!!!!

Replace your exhaust manifold now!

You really have to watch the head gaskets on Neons. That would certainly be the first place I'd check.