Need technical know-how / advice!!

[Captain KRM P5]

I am looking into buying a 323 GTX AWD, and the seller has said that there is the following problems;

The car, as it stands, does not run reliably. The car
has a problem maintaining oil pressure in the cylinder
head, and only in the cylinder head.
I've replaced the cylinder head (when it ate the first
one), and the block. The block has a new oil pump and
water pump. Block oil pressure is 50-75 PSI, but
cylinder head pressure starts at 25PSI (within spec,
due to a restrictor orifice) but drops to below 10PSI
(out of spec) after about two minutes.

and further more;

Truth is, I don't know what it would take to get rid of the oil pressure problem. I've already replaced every part in the oil system. (pump, block for passages, main bearings for too great a clearance, head for too great a clearance, etc.)

What kind of problem am I looking at and how much $$$ to fix? Thanks people!