Servocity.com do a wide range of geared motors that will work well for peristaltics.
The bigger you can make your pump, the better. Also don't forget you can make the rollers deeper and run two or more sets of tubing, to parallel up the flow rate.
Most designs use no more than three rollers for efficiency. You can get a rough idea of the amount of water your pump will flow for a certain RPM, by dividing the circumference of the tubing diameter by two, then multiply it with the cross-sectional area of the inside tubing, and multiply that by the RPM. This doesn't account for the tubing pinched up by the rollers, but it does give you a clue that you're moving in the right direction.
A pump which is 5cm diameter with 4mm tubing operating at 250RPM will pump about 250ml a minute.
The flow rate of these pumps is typically very poor when compared with geared or centrifugal pumps. They're primary advantage is that they self seal, and are easily rebuilt.
Doubling up the tubing will double the flow rate, assuming your motor can maintain the same RPM. You can also increase RPM, however be careful running too fast as this will reduce the life of the tubing. The service life of a pump like this in a model is likely to be considerably extended over industrial applications, but a burst pipe could have unpleasant consequences for the longevity of your sub! You could fit the pump head in the free-flooding space. This serves two purposes- it removes any heat put into the tubing by the action of the rollers, and should the worst happen and the tubing split, the ballast tank will flood, but the dry spaces should remain intact.
Be sure to fit a pressure switch in or some sort of mechanical relief valve. Whilst other types of pump will tend to stall, perstaltics are capable of reaching dangerously high pressures unless chcekced in some way, so it's important to have some means of purging the pump should it get stuck on for some reason. Norbert Bruggen sells pressure switches, and the Thundertiger Neptune also has one fitted as standard equipment, so it should be available as a spare part.