That's not the original Darnell system, the previous owner has done their own thing. The original Darnell system used a small steam pump, which moved air from flexible bags to a plastic bottle/reservoir mounted in the forward compartment.
A lot of people modified the basic system by getting rid of the steam pump, and fitting a small air compressor, the type used to inflate car tyres from the cigarette lighter socket. This improved dive times.
Darnell models have also been adpated to work with pumpe water ballast systems piston tanks etc.
There is no one 'perfect' ballast system, they all have pros and cons. Some are less expensive than others, some need special tools to build etc. etc.
The system you are proposing is the same as that used in the Sheerline range of model submarines. Water is pumped into a sealed tank, and the air compresses above it. A pinch valve prevents the water shooting back out of the pump when you stop pumping. This system is simple and inexpensive, but there are a few things you need to consider. First of all the tank must be completely airtight and of sturdy construction to take the pressure. Some people fit an auxialliary reservoir to help keep the pressure inside the main tank within reasonable limits. Aerosol tins are good for this, as they're built to take pressures much higher than we will require. If you retain a single tank, then a piece of PVC or acrylic pipe is very suitable with plastic endcaps bonded at each end, and held together with three or four threaded rods.
As the tank is only partially filled, it must be baffled to avoid the water sloshing about.
The pump you choose must be capable of the reaching at least 40psi if you're to achieve a reasonable fill of the ballast tank. Geared pumps work very well, although their flow rate tends to be on the slow side. Kavan produce a good quality pump suitable for your size of boat. They cost about £25-30 (try ebay) new.