03 Disco Overheated - Where do I start?

Hi All,

I've been lurking around here for a while equipping myself with as much knowledge as I could possibly find, and now I need your help.

I bought an 03 Disco about 3 months ago with 104k miles on it. I knew that I would need to start performing some overhaul on it when I bought it so that's not an issue.

The truck ran great when we got it, but a couple weeks ago, I noticed it took the heater forever to get warm. Then the following day we took it out for the morning, and this time I left the heater off, assuming I needed to wait for the car to warm up. Well about 2 miles down the road, the temperature gauge started creeping up ever so slightly past the middle mark, and I immediately decided to turn around. We drove another half mile and the temperature was closing in on the red and the red light had just turned on, so I immediately pulled to the shoulder and left it off for about 10 mins.

After that, I started the car, turned on the heater, and made it back to my garage without overheating again. I feel like having the heater on from previous trips is what kept us from noticing this problem sooner. Once in the garage I noticed that almost all of the coolant was gone out of the expansion tank, with just a little bit at the very bottom. There's no visible coolant spray from looking under the hood, so I fear that it's the head gasket.

I was already planning to replace the radiator, it needed new valve gasket covers, and was planning to do the high mileage service based on the list in the sticky of this forum.

So my questions are -

1) Does this sound like headgasket failure or just radiator failure? or both?
2) Should I just go ahead and replace headgasket considering mileage?
3) Is there anything else I should be looking for that might be contributing to this problem?
4) What are the odds my heads aren't warped? It was only in the red for about 1.5 seconds before turning off the engine.

It looks like it was running OAT Orange Coolant. And the general consensus on here is to use Peak and avoid Dexcool. So I plan on flushing everything, replacing radiator, and starting over with Peak.

I'm also planning on addressing the lifters, as it would occasionally give me some lifter tick at idle.

So this is my grand plan unless I am told otherwise. (In no particular order)

Flush entire cooling system out, Replace Radiator, and Expansion Tank Cap - Refill with Peak
Replace Head Gasket
Replace Valve Cover Gasket
Replace Waterpump
Inspect Cams and Lifters Repair, Resurface, or Replace as Necessary
Inspect Heads for Warping - Repair as needed.
Perform High Mileage Maintenance as Outlined in Disco Mike's Sticky Thread

I'm fully prepared for the money, and time commitment - I'm pretty mechanically inclined, and I have my RAVE manual all outlined. Just looking for advice on any pitfalls and things to look out for during the procedure. And if there is anything else I may need to do.

Thanks in advance for any help! It's much appreciated!