Remember me? "No start under 5 degrees C unless plugged all night"...

I had tested everything electrical component and my mechanic replaced a faulty check valve last week, but I still can't start when it's cold (not even freezing yet...). Most people in the group tended too agree that it was likely the cold start injector was at cause... only problem is LH-Jetronic 2.2 doesn't have a cold start injector! Yesterday my mechanic suggested a vaccum leak through the head gasket. I thought it wasn't making much sense but my Haynes clearly mention this as a possible cause of my problem. Vacuum leak could also be elsewhere though: oil fill cap, dipstick, hoses, conduct...

My questions are :

-Is this diagnostic realistic?
-Could I have caused this leak by driving on compression a little too often (I love to use the engine brake to slow down ;))?
-Although the cost of having the head gasket is reasonable, is it something I could easily do? At this time of the year, I would have to rent a garage and be on my own if I happened to goof: no car to go to the auto part store, etc... ?(it happened before: I took the driving shaft of to replace the central support bearing. The bearing they had sold me turned out to be the wrong size! It's late, I need the car in the morning and I don't have transportation to go back to the god**m part store).

Thanks again for your help!

Richard Bouchard.