Water hits boiling point quicker than coolant and leaves a residue in the working components coolants work at minus 25 degrees and plus 160 degrees and keeps the engine well maintained and therefore never reaching boiling
you really shouldn't be putting just water in you should be using a good quality coolant , is there any damp on passenger floor well the only other thing could be a leak at the back of the engine going straight onto the exhaust burning of so there would be either no wet patch or very little
Just looking at your profile and have to agree I keep buying these I've a 51plate Serengeti, 2x54plate one standard and one HSE and Friday picking up a 56 plate sport auto that's a one owner 68000 mile, FSH Can't get enough
Regarding lack of power have a look at the pipes coming of the vacuum selonoid at the front under air intake they are prone to melt and split you also might be needing a new low pressure sensor on the end of the rail that's the cheapest option
The clunk could be the bushes on the diff one on each side and one at The back , their easy to change up in the air but a bit fiddley on the ground you don't have to take the diff off
There's loads of threads about fuel and starting problem's , you could crack a injector pipe open turn the engine to see if fuels getting through then go from there
Could be one of the vacuum hoses or a split turbo pipe , try to find where it is by running your hand where you think it's coming from you should be able to feel the air against your hand