Each starter draws about 15 amps max.

Maybe a 15 amp breaker for the starter control circuit. A starter is
series wound and will draw the current it needs straight from the
battery. I would think that several hundred amps peak would be
normal. The starter/generators on small turbine engines draw as much
1200 amps when you hit the button then taper off as the load
decreases. I've seen GPU power cables jump 6" off the ground from
the initial surge!

If you want to use a power supply to keep the battery from running
down while the aircraft is powered up in the hangar, most any source
of DC capable of putting out the amps drawn will work. What I don't
know is how well one of those converters filters out ripple. Since
one end is plugged into alternating current, unless there are some big
capacitors in there, you will get pulsed DC out the other end. I
don't think that would be good for the avionics, but I imagine that
the battery in the circuit would smooth things out.