Hey Q, you have two choices here. One is to get a panel that does not have the capacity to overcharge that battery. Basically, if the panels max amperage is below 2% of the battery's amp hour capacity, then it can be hooked up continously without causing any damage. In your case, 125 AH X 2% = 2.5 amps. A 50 watt panel would be the maximum size you would want for this. Instead of a charge controller, all that would be needed is a diode to keep the battery from discharging through the panel at night. Most panels today come with diodes already installed in the junction box, but if yours does not, they can be had at Radio Shack pretty easily. Look for a 6 amp silicone diode for about 79 cents.
That size panel would take 25 good hours of direct sunlight to bring your battery from half full to full however, which may not meet your needs. 25 hours of good sunlight might take 5 days of pretty dry weather. If you need it to recharge faster, you can go with more solar, but you'll need the charge controller now to keep from overcharging your battery. The 50 watt panel might cost you $125 USD, a larger 120 watt panel about twice that amount, plus another $75 or so for a controller, something like a Morningstar 10 amp would work. The 120 watt panel would produce up to 7.5 amps in direct sun, which might bring your battery from half to full in one good sunny day.
There is a great book at the library I would suggest reading, it's written by Richard Perez, who also happens to be the founding editor of Home Power Magazine, the only periodical that really gets into the nuts and bolts of renewable energy. I would suggest ordering the mag and checking out the book myself. We subscribed 13 years ago, our home is currently powered by the wind and sun. Most of the equipment we put in ourselves over time, so I fully understand how it all operates, that is where that magazine and some other sources have gotten us. Whatever you do, remember to keep that battery watered, at least once a month in warm weather. Take care Q, Rudydoo
The Complete Battery Book, by Richard Perez, (library)
Home Power Magazine, Homepower.com
Solar Energy International, Solarenergy.org
Midwest Renewable Energy Association MREA.ORG