That is a vintage Sportways manifold. Probably from the 70's. It is a good manifold, but it is not what most people use now a day.
I don't feel a real need for isolation manifolds except in overhead environments, but I service my own gear and I pick what I feel is some of the most reliable gear.
The tanks do have the standard 3/4" NPS threads, so replacing the valves is not a problem.
The price (just for the two tanks) seems a bit on the high side, but depending on your local market it may be fair. I am assuming that you may want to replace the manifold. That manifold itself is not worth much in today's market, but it is perfectly usable.
Doubles do balance better than a single large tank, but are going to be heavier out of the water and the maintenance of two cylinders versus one large could be basically twice as much.
I can't tell from the picture, but if the tanks are not galvanized I would not even think about them.