"I want rock sliders that are attached to the frame, not just on the corner of the tub, but I don't want to go nuts with something that's overkill for this jeep."
I thought that as well - but with the early cj5 - the through the floor pedals and frame mounted master cylinder make that almost impossible - so I went with body mounted ones and they have held up very well - rubicon bashed and thrashed. You can high-lift jack them as well.....of course, frame mounted ones are also great. I wouldn't rule out body mounted ones though unless you want to tie your roll cage into your frame and sliders. And there are some weight savings, and a bit more ground clearance to the body mount style well. I think a model 20 with single piece axles is plenty strong for anything other than full locker, and even with a full locker, it would probably be okay with 33s and 258. They guys blowing that stuff up typically are running v8s and 35s etc. How easily something blows up is often related how aggressive the driver is with the skinny pedal. As you mention, there is a lot of balancing between what you have, what you want, what you can afford - how it all fits together etc....
"Rear lockers will push, making sharp turns more difficult, and (as you mention) the AMC 20 is not very strong. Front lockers will help to pull the heavy front over an obstacle, which is more effective than pushing from the rear."
I think a LS in the rear and a full locker in the front will get you most anywhere. But it is my experience that a single locker in the rear is better than a single locker in the front, or even LS in front and rear. When climbing slick steep spots where the locker is most needed, all the weight gets shifted to the rear axle....locked up wheels will still spin if there is no weight on them, and this is much more pronounced in short wheelbase jeeps....I also think affect on steering a rear axle locked up is pretty minimal (and I run manual steering), but no way I could turn with a front locker (power steering of course makes that less of an issue). Again, a LS rear, locked front combo will get you almost anywhere....you can always add a rear locker at a later point, and I get the need to minimize the stress on the model 20.
"I'm hoping the T4 will be adequate,"
That would be my single biggest concern....but I also am one that kind of uses whats there until it breaks, then replace it with an upgrade. Downside to the T4 is weak transmission and no overdrive....in case you want some replacement options to look for - the T176 is a solid 4 speed (no OD) that would essentially be a bolt up option as it was offered OEM on some cj models. For a five speed with OD, other than the T5, the later ax-15 is great choice, but it would probably require quite a bit more work to get in. I am fighting some of these same choices with my 58....thinking of replacing the oem 3 speed, but it gets complicated fast with various adapters and modifying cross-members etc....I keep waiting to blow up the T14 in my 70, and then it'll be replaced with a truck 4 speed, but it goes and goes....I spend more time researching and investigating options than I do actually working on it...As I said, just some thoughts from my experience...your overall plan is solid, and probably overkill for most moderate and overlanding style wheeling. It's going to be a pretty sweet jeep and extremely capable.