Cap gunk is normal especially if you have non-GM oil and/or do mostly short journeys .
With a W reg it is quite possible that a coolant pipe somewhere come have a small hole (which have a habit of shutting closed when the coolant cools

) and if it is in an area where there is something hot like an exhaust it could evaporate before it hits the floor. I have a lot of experience with coolant leaks, if you search for my thread on swapping the heads on goldie you'll see why.
I would do the following:
FIRST: You are looking for leaks with a hot fluid around a very hot engine so gloves and protective equipment are a must.
- Check the coolant/water mix. This will assist with finding the leak.
- get the engine warm by going on a spirited run to get the coolant pressure high. Pop the bonnet and see if you can smell coolant/mositure. If so hen the leak is near something hot. If this is the case then the following are suspect:
- oil cooler plate
- coolant bridge or it's pipes
- thermostat gasket
- coolant transfer pipe (fits in to the thermostat housing)
- coolant pipe gasket (at the rear of the engine block)
- If you cannot smell any coolant then you need to get a torch and with the engine running gets eyes, and hands if you can, on all the pipes. Squeezing them can sometimes make more come out which will help you id the leak. This will involve getting all around the car from above and below so ramps may be necessary.
- If you are confident there are no leaks in the pipes then we are in cyl head territory. A pressure test will not always give you the result or all the answers as many sides of the head are not visible, you may just get 'there is a leak' rather than 'this is where the leak is' but it would help you further diagnose the problem.
My heads were shot but the fire rings were fully intact. This was caused by the previous owner driving with a blown oil cooler (short distances, topping up the coolant as he went) this corroded the HGs and blocked the cooling passages in the heads.
I hope this helps
