Just in case readers are looking to undertake a similar operation, I wouldn't exclude the Chemco surface tolerant primer on the basis of cost. Given the massive effort required and/or cost involved with preparing the steel, a primer that covers and seals as well as the Chemco product, although pricey, may be seen as excellent value, especially in hindsight!
When my boat was ashore in the boat yard, there was an old steel motor launch next to it and he prepared the hull to a fairly high standard, but by the time he got round to priming it with Chemco primer, it was brown with rust all over once more. The chap arrived with the two pack product, a tray and rollers and set to work. After an hour or two it started raining and he continued to paint the wet, rusty steel hull. The rain worsened and the paint in his tray began to cover with rain water. He carried on, pushing the roller through the water to pick up the primer. I watched with interest as he finished painting the wet hull and drove off. The boat looked great in its new silver/grey livery, but I thought to myself, it'll never last.
That boat remained untouched in the yard for about three years and I kept an eye on the paint job all that time. It only had a single coat of primer and in those three years it never had a top coat, but when I saw it last there was no sign of rust anywhere.
If anything can be learnt from this, it is that if you are not sure about how well you are going to be able to do the preparation work, or simply don't intend to spend the time necessary to achieve a perfect finish, buy the best surface tolerant primer.
I applied the Chemco glass flake primer/topcoat combination to my barge in 2011 and there is zero corrosion anywhere so far. The manufacturer suggests it could be good for 25 years and given its performance so far, I wouldn't be at all surprised.