Just a post-trip update for posterity.
I ended up spending more than £2K preparing. Almost half went to an Ohlins rear suspension which I am moderately happy with.
I got the GPS and am happy I did - it's extremely useful in cities & deserts and marginally useful anywhere else.
I got a smaller mattress, tent & sleeping bag. Very happy I did as the end result was that I could pack everything inside my panniers (protected from water, thieves, accidental loss, dirt, sun, better aerodynamics etc)
I spent money to ensure I had the right stuff with me, and not just something that might do the trick. I used every single item in my panniers at least once, save for the tent repair kit (which came with the tent anyway) and the aluminium ("space") blanket, since I never got myself in a survival situation. I also didn't use some of the drugs I had with me, but I won't complain.
Didn't get a new jacket. Didn't need it. I had an accident, was knocked over and ended with the bike on my foot - the jacket protected me. I had bought new boots (not off-road, but middle-ground road type ones with tough ankle protectors and fully waterproof) and they kept me dry and saved my ankle when the bike fell on it.
If I were to leave for a trip tomorrow I neither need or want to buy anything. I know the equipment now, and I know it works for any situation I'm likely to encounter. So the mix'n'match approach between old and new was the best for me.
I'd say buy as little as possible new gear, but whatever you do purchase, make sure it's top quality and it fits your needs. You will be using it again and again and again...