Was the back pressure you're talking about showing up when you have the two pipes off the oil cooler connected together? It's hard to say whether it's the gasket aswell as the cooler. Extra coolant pressure in the system can test seals everywhere. Classic symptoms of head gasket blowage are steam from exhaust when hot, oil in water, water in oil, or in my case bad misfire and a cylinder full of water! Cracked the spark plugs when hot and all but one were good - the odd one out was wet and steam came out of the hole in the head.
Once you've flushed it, it might be fine, but i'd run it for a while with a new oil cooler and test for hoses going hard straight away when the car is cold. Head gasket job is a 3* in haynes - take the timing belt off, the manifolds (either when the head is still on the block, or afterwards - need to disconnect exhaust for that) undo the 10 bolts in proper order and the head should be removable. I'm in the middle of my scirocco at the moment! Rusty bolts aplenty..