Is the spool mount k-member worth modifying/reinforcing and what should I change on the suspension brakes?

The K frame is absolutely worth reinforcing. I fully seam weld mine, as well as boxing in the steering box mount and adding reinforcing plates to the ends of the LCA pivot tubes. Some of that is probably overkill, but the steering box mounts on one of my '74 K frames had cracked and broken loose from the rest of the K frame which allowed the entire steering box to move around. So I would say that boxing in the steering box mount and adding the plates to the LCA pivot tubes is absolutely worthwhile for pretty much everyone. Additional seam welding and reinforcements are good too, but more relevant if you're going to run more of a pro-touring style. This thread has a bunch of pictures of reinforced K frames

k-frame strengthening

Drop spindles are a complete waste of time if you're planning on running larger torsion bars, even if you plan on lowering the car. The drop spindles are all a 2" drop, which is pretty substantial. If you use those spindles and don't drop the car a full 2", you're increasing bump steer and reducing your camber gain, both of which will be noticeable. I ran 2" drop spindles on my Challenger for awhile before I knew better, they make for worse suspension geometry than just lowering the car with the torsion bar adjusters and using the larger torsion bars to account for the loss of suspension travel. I wouldn't go smaller than PST's 1.03" bars personally, I've run 1" bars and even those are fairly soft with modern tires.

The factory 2.75" piston calipers have more clamp force than Wilwood 4 pistons by the math. And they have seals and were designed for prolonged street use. The upgrade to the 11.75" rotors is a good one, use a set of 76+ 2.75" piston calipers and you're good to go.

Proforged is making decent replacement suspension and steering components, they don't make everything but I would use them instead of Moog if you want to keep rubber bushings.