2000 Disco 160k-Love/hate relationship

I need some help getting back on the road with the least amount of money out of pocket. I have had problems since day 1 and the check engine light has been on ever since I got it with 60k 6 years ago. They said it was a sensor between gear 3 and 4 and I just didnt get to it due to all of the other problems. One good thing is this thing always starts. Right now it is leaking from under the water pump which will be changed after all other avenues are exhausted. Head gasket replaced 2 years ago but it was done by an idiot. Thermostat was changed because I removed it and found pieces falling off of the plastic. It was running fine thern the heat went out and the temperature climbed above the mid point for the 1st time. by the time the light went on truck was off. Let it sit and filled the coolant to get home since it was snowing and I was far from home. Next day same thing. I am going to flush this out since the plastic may have clogged the heater core. Can you also tell me where the heater core is and what it looks like? Learning. Other issues-3 amigos, o2 sensors replaced too many times and still done work, 10 miles to the gallon, black smoke when I rev it up and black spot on ground, possible slow trans leak. Where should I start and if you know estimated part prices it would help? Thank you for your advice, this site has helped me get this far.

 

Step 1 is buy an ODBII scanner, you can get some cheap ones at HF or Walmart, or better still get an ultra gauge www.ultra-gauge.com
Step 2 is read the codes and post them here - much better than guessing.
I suspect the "sensor between gear 3 and 4" error could be your MAF, but without codes that is a guess.
Are you leaking oil or coolant? If you are leaking coolant from your water pump then your water pump is toast and you need to replace it asap, before you cook your engine.
The heater core is located in the dash, you cannot see it. If you look in the engine bay you will see 2 black hoses going through the firewall on the passenger side and they are connected to hoses that run along the passenger side valve cover. When you flush you can disconnect the hoses and flush water both ways to check if the core is blocked.

 

It is leaking coolant and I will get this flushed and a new pump put it. If it drives after then i will get that tool for the codes. I just dont want to buy a pump put it in and find out it is something else. Are there any things I can look for to see if there is a cracked block, heads or another blown head gasket? I may be crazy but I think there is coolant around the gasket for the valve covers which is scary but she is a trooper and I know she can handle a little heat. lol