Have you dropped the trans pan and replaced the filter yet? Might be something as simple as needing new fluid and a filter. Dropping the pan with also give you an idea as to how much clutch material is in the pan. *take and send photos*
You might find this an easier task to complete if/when you remove the exhaust. Otherwise just be prepared with some penetrating oil and new exhaust hardware.
Have you drained the front/rear Diff and the transfer case? If not, I would do that asap as well.
Regarding powerstop pads and rotors…they work very well for our heavy ass vehicles. I’ve used the rotors on my 02’ but have no experience with their pads. @Doubeleive can chime in on this as well.
One thing I didn’t notice if you or anyone else had mentioned were the fittings for your coolant lines. The plastic quick connect fittings are notorious for being brittle, especially on an older vehicle. Have them on hand when you go to make that swap.
I see they haven't responded in a bit but funky shifting in a otherwise working transmission could very well be a bad maf or solenioids
at 274k if the maf is original throw that thing away it's pretty typical to see them start being weird at around 160k and they can cause all kinds of seemingly unrelated problems.
as far as brakes a good premium ceramic pad has always worked well for me I am very ******* brakes, rotors are rotors unless you are towing a lot or driving it like it's Porsche then you might consider better stuff there are many brands out there and many opinions drilled and slotted may be less prone to warping but some cheapy brands will tend to develop hairline cracks, if you want to go extreme there are carbon fiber options but they are noisy and make a ton of dust. or just buy a big brake kit if you are flush with $$$$'s
I've had great experiences using NAPA Gold premium ceramic pads. The NAPA Ultra Premium poly coated rotors were awesome! I had them on my Jeep for three salty New England winters and they still looked brand new. No warpage, they're beefy, great stopping and not speck of rust.
I am doing Raybestos R series on my 04 Tahoe now, because funds are tight now.
I definitely have the NAPA coated rotors on my list for next set of brakes. With those, I would expect to change pads many times before having to do rotors again.
Whatever you end up with...
In the long run, it definitely pays to not cheap out on rotors.
First thing I would address is the brakes, you could maybe just need to flush and bleed them real good but inspect and if need be do pads and rotors at the same time
I would then move on to the cooling system, temp should be rock solid under normal conditions and should not fluctuate at all, being high mileage and unless you have evidence that it has been replaced in the last 5 years I would just replace the radiator and flush everything out real well, use some radiator flush first with just water and run it a couple days, drain and refill with water a couple times afterwards then use 50/50 coolant/water mix for the final fill. this should resolve the temp fluctuation if not then you need to move on to the thermostat and water pump.
you may want to invest in a good scanner like a tech2 it's what the dealers use, you could use that to see what is going on with rich mixture and go from there. cat's are pretty pricey right now due to covid and production being slow. so I would try to stretch that need out until the prices get better again. just for reference cat prices are double what they were pre-covid.