Hi,
Well, the PC power supply can be modified for 13.8v but it won't supply 25A. So, you have to build it from scratch. I would say use an SG3525, IR2110 and drive 2 high-voltage MOSFETs in half-bridge configuration driving a ferrite core. The output would be a full wave rectifier with schottky diodes and a feedback stage. The circuit should use a shunt and have short-circuit/overload protection. I'm assuming you're going with a float charger. In that case, all that is needed is to use the feedback stage to keep output fixed at 13.8vdc. If you've gotten that much, I'll explain further.
Hope this helps.
Tahmid.