I have to agree that if the car is tatty and with gearbox being at high risk, it's probably not worth spending over £2k.
However it does not have to be 2k for blown gasket, you just did oil cooler pipe replacement yourself so clearly you can do the spannering yourself. I would at least get the head off and inspect the damage and possibly attempt budget repair if the head are OK and gasket looks to be the only issue.
You only have to do both sides if you are skimming a lot(over 0.1mm) off head due to damage or warped head as this would cause difference in compression ratio between banks. Normal skimming is only couple of thousands of mm and will not make much of a difference so you can do only one side or if the head is straight and without damage to the face you can get away without skimming at all. You can do basic checks for the head visually yourself and with the help of bit of chalk and kerosene or diesel check for cracks or leaking valves.
Gaskets does not have to be £200 per side if you shop around. Below is the screenshot of example after a quick search if doing one side only. EUR146 or EUR 106 if you reuse old bolts. Posssibly find them even cheaper with a bit more searching.
Cambelt, rollers and water pump - depending how old they are, I would be tempted to change just the belt if bearings in rollers and pump are OK and without play, possibly put a fresh grease in bearings. So it is possible to turn it around for couple hundred instead of couple thousand.
