After sitting in my backyard fir over 15 years thought it time to remove and replace metal. Keel has 6000 lbs lead. Used tire weights with metal clips melted off poured in.
The steel doesn't look all that bad , the inside looks great. Your first step in working on that boat should be to get some 35 inch high, solid lifelines on , whether temporary or permanent. Its a long way to fall and you wont have water to land on. I once fell off a boat and broke my wrist and pelvis, a real pain in the ass, literally. Luckily the keel was not on yet. Had the keel been on, I would have been killed. Having good lifelines feels a whole lot more secure, when working on deck, allowing you to concentrate on what you are doing.
Stainless is far harder to find in scrapyards than it used to be. If you use stainless for the first 4 inches above the decks, and go galvanized from there, you can easily change it all for stainless later, when some shows up, without burning any paint or foam.I have used galvanized sch 40 pipe in the 80's which, well painted, looks as good as the day it went on.
I'd leave that transom off as long as possible, for easy access and less height to climb. If you have any sandblasting to do inside, it gives you a place blast the sand out. Cutting holes in the hull at low points, also helps. Other wise the sand builds up faster than you can blast. You can then blast the cutouts and weld them back in, then blast the welds.
After you have thoroughly epoxied the inside you can put the bulkheads in, far easier than putting them in after foaming. Putting the transom in takes only a few minutes. Put it in over size, and fully weld the inside, before cutting the excess off the outside, grinding it flush, then welding it.
Making the bottom sharp corner of a reverse transom, where it is impossible to keep paint on, out of stainless, will save you a lot of maintenance problems later. Ditto trimming all outside corners with stainless.
Wheel balancing steel clips are 20% of the weight They all float to the surface where they are easily skimmed off. Welding a plate over the ballast keeps oxygen out, eliminating any electrolysis between the lead and the keel plate. As an added precaution ,you can weld a stainless nut to the top, drill a hole thru it , pour a bit of oil in, then put a bolt in and weld it airtight. With a layer of oil on top, there is no chance of oxygen getting to the metal