7.25 rear woes....

My ignorance is also a crutch.

Saw this just now
[FOR SALE] - 1969 A Body 8 3/4 rear end

Does this discription mean i would need to buy the “chunk” or “pumpkin” or center piece or whatever and can i put disk brakes on it?

Is this the kind of thing i should keep my eye out for or a complete one?

Yes you would need a center-section
I wouldn't bother with retro-fitting rear discs brakes. Typically the rear brake system does less than 15% of the total braking. Even small Drums can easily handle that.
There are two things that kill a 7.25; peg-legging it and shocking it. Three if you include old contaminated oil. If you put a SG in it, that kindof eliminates the former. The auto sortof eliminates the later. If you can refrain from doing neutral drops, you might be surprised how long the 7.25 lasts. I had one for years behind a 340 auto.
245s are about the biggest tires you can put under there without moving the springs and getting custom wheels, so that is a non issue. Even less of an issue if you put them on the factory narrow wheels