So the p38 drives very well and doesn't overheat. Good start.

Needs 2 new front air bags... How much should I pay for them each? And if anyone has a link would be great!

Needs a new abs accumulator. Again any suggestions and what price is acceptable?

But today I noticed the coolant was down about an inch. There is also residue around the filler cap and there is a residue below it down to the floor. By residue I mean white stain dried stain. When I test drove it I got it upto temp and it was perfect. All seems fine still and temp stays fine even on idle. There was pressure in the header tank but no more than there should be. There were no bubbles and no sign whatsoever of oil in the water... So what's going on?

I got mine from Island. 50 quid a corner plus VAT for the Dunlop ones. or 4 for 187+VAT.

http://www.island-4x4.co.uk/springs-dunlop-isl004-p-628.html

Mine had had 2 sh!tpart ones fitted the year before I bought it and they were already perished so I wouldn't economise on something so important.
Again my ABS Accumulator was from Island. I use them mainly because they do a good delivery service to Czech republic and have always been good. you might be able to find things a bit cheaper but not much. (I've heard they're impossible to get hold of by phone but I had a query on my last order, sent them an email sunday evening and had an answer by lunchtime Monday so very happy with that.

Might just be a weak pressure cap releasing a bit of pressure when you stop and bits of water start to boil up against the hot engine. If all else is well, I'd try a new cap on the tank, doubt they are super expensive.
Other key leak points for slow leaks are throttle body on thor engines and heater matrix o'rings

http://www.island-4x4.co.uk/expansion-tank-febi-pcd000070fb-p-31088.html
about a tenner for a cap with VAT

http://www.island-4x4.co.uk/pump-accumulator-wabco-stc2784-p-1024.html
ABS Accumulator, 85+VAT

That was prompt and very helpful!! Also much much cheaper. Garage quoted me £120 + plus vat per air bag!!

Also what's a thorough engine? And there is no water in the drivers footwear so hopefully other the o ring.

120 quid might be including fitting. A bit on the steep side I would say but not an unusual rate if they're allowing an hour per corner.
The "thor" engine is the later version (approx 2000 onwards I think) of the V8 with the bosch engine management. The "gems" is the earlier lucas based system.
Gems has a big rectangular box on top of the engine, thor has a bunch of bananas on top.