So huge coolant leak out the rar of the engine this morning. Only 105,000 on my 2001 disco. So that sucks.
What does a good shop charge for the job? What add-ons should I have done at the same time?
I'd really love to plop a 4.6l in this thing, and this is certainly an opportune moment to consider it. Or convert it?
What should i do?
Or just trash this thing and move on? Not really looking at this route since it's decently solid and lifted with custom stuff.

 

Have you confirmed that the leak is coming from the head gasket? There are other places for coolant to leak at the rear of the engine.

In general, unless you're dead-set on putting a 4.6 in the vehicle, it won't be financially justified over simply repairing your 4.0. You'll never see any money you spend on conversion come back to you, and frankly the 4.0 vs 4.6 is not a huge difference in driving experience.

Head gaskets (if they're in fact bad) can be replaced for a couple of hundred bucks if doing it yourself. A local mobile mechanic on craigslist (Los Angeles) advertises for $800 for the complete service, including parts. Bring it to a decent independent mechanic, and you'll spend ~$1000.

I'd just figure out what's leaking, fix the problem, and get back to driving it.

 

Fix it yourself.
All you need is a parking lot and tools.
you can even work on it at night.

Here is my adventure fixing head gaskets on a 2001 D2
in a parking lot. No power even.

I used a battery and an invert-er and a CFL in a trouble light:

https://www.flickr.com/photos/76579732@N07/

 

When you are done with your head gaskets and all proud - clean up the engine
with GUNK and 3M engine cleaner and it will look like this and run great: