Hello everyone!

I am new to the Land Rover community. Excited to be here!

I recently purchased a 1996 Land Rover Discovery SE7. It runs and I hope to use it as a daily driver but here some of the high priority issues I am up against and hope to fix soon:

High Idle:
It idles anywhere between 1800-2000. I've read around the forum and i've heard suggestions of checking and/or replacing different sensors (MAF, IAVC, TPS, ECT). The guy I bought it from recently replaced the MAF, IAVC, and the TPS. He did mention that he thought the ECT is bad and/or the thermostat is stuck open and could cause that idle, which I have also read. I hope to check all these sensors soon to rule bad sensors. I also plan to check into the throttle cable and see if it needs adjustment.

My question: Does the ECU need to be reset given that all these sensors were replaced? I've read that people have replaced these sensors and then they simply had the ECU reset at a local LR shop. It's not throwing any codes right now so I have nothing to check. I am getting a OBD2 scanner in the mail tomorrow and hope to check the temp.

Transmission:
I have experience working in a shop but my experience only goes so far so when it comes to transmissions, I am somewhat clueless. Here is what I am experiencing: When I put into D, it starts out in 1st gear. It will shift HARD into 2nd but my rpms will be pushing 5 and I almost need to toy with the throttle to get it shift. Then it won't even shift into 3rd or even 4th. This meant driving it at 50mph all the way home when I bought it I was able to get it into 3rd once (not sure how) but when I was going up a hill, it shifted back into 2nd and I never was able to get it back to 3 (and of course not even 4th gear). One thing I noticed is that when I am in second, it doesn't even allow me to shift down to 1. I drop the shifter and it doesn't do anything.

My question: where do I start with this transmission before just buying a new one? The shifting cable seems off when I am moving the shifter. It seems like it needs to be adjusted. I am considering new fluid and filter. I have also read about xyz switch that can get clogged. I basically want to start with my cheapest options and move from there.

General Maintenance:
It's been sitting for a while but in terms of the engine, it runs strong. It has almost 160,000 miles on it. There seems to be a miss and it backfired a couple times when driving it home (I think this was due to the high idle and it not shifting). Are there any specific things about Land Rover D1's that I need to perform maintenance on? Any preventative things I can do? Recommended oil for these motors?

This is my first post and it's a bit long. If you think I should split these three different categories into three different threads, let me know if you think that is best!