OK, firstly thanks to @mudski for the help he's given me so far.

I'm still learning this all as I go, so I may have some strange sounding questions!

So I want to fit a larger exhaust to my 2013 3L CRD Patrol with 78,000ks on it.

If I find my OEM intercooler is leaking, is it worth repairing or should I replace it? Are they easy to fix? Should just about any radiator place be able to fix them?

Setting a Dawse valve. From what I understand, the Dawse valve relieves pressure when that pressure reaches a certain point. How do I know what that point is? I've been told fitting a Daws valve isn't hard, but I have no idea how to adjust it or what to adjust it to???

Needle valves. I've read that they can be used to alter the speed the turbo spools up at (not sure how, haven't looked it up yet). As I don't fully understand it's function yet, can I do any damage if I alter the needle valve setting? I understand it's there so you can get better response or economy from your motor, but is there any chance if I set it for one thing, let's say driving at 100kmh on flat roads for hours and then go belting up & down slippery muddy tracks, could I do any damage? I'm thinking about EGTs here.

Gauges. Does anyone know of a small combined boost & EGT gauge that would fit on the bottom in between the taco & the speedo. I can't use a pillar pod as I have my mobile phone dock there.

The input to the gauges, the stuff that makes the needle go up & down, is it air, fluid, oil etc or electrical? I'm wondering what will pass through the firewall and how large a hole I'll need.

Dyno tuning. Is it worthwhile if you don't have a chip? Will my Dawse be set as well as it can be if I get a dyno???

Exhaust. 2.75" or 3"? I've read that all that is needed is the 2.75", but there are many more options (and prices) in the 3" world.

EGTs. What is too low and what is too high? Does adjusting the needle valve have any effect on EGTs? How do I stay safe?

I'm after a reliable efficient vehicle that will last me for many years, I'm no mechanic so I need things to be robust. if there's any advice or info on what else I should or shouldn't do I'd greatly appreciate hearing about it.

Thanks for any help you can offer.