Best to measure the load with a meter. Most are rated at 10Amps so have a spare fuse, though I doubt the setup is pulling more than that. For example, My home pc has a 300 watt supply , p4 , tv tuner card , dvd drive and I cannot get it to draw more than 100 watts.
If you do add a psu ( I am considering the same ) you could get away with a battery charger from an auto parts store. I've tried this on the garage floor and it worked well though kind of ugly. This kind of charger could be permanently installed somewhere with adequate ventilation. BTW I could not get it to work with out a battery in the circuit.
One supply I really like here..
would do nicely if I go back to a dual battery.