Wow, nice score on getting that panel for only $100!!! I'd recommend against doing what you proposed, however, as you'll end up with a lot less useful power than you might think. Three things to consider:
1 - Right now you have (6 * 15) + (2 * 64) = 218 watts. Putting 3 of the 15 watt panels and 1 64 watts panel in series will give you 48 volts, but the amperage will be limited by the smallest link in the chain which is the 15 watt panels that can only handle a nominal 1.25A. That means each string gives you 48 volts at 1.25A, or 60 watts. Two of those strings gives you 120 watts, so already you have lost almost half of the power. Paralleling these strings with the 50 watt panel you got gives you 170 watts max which is 48 watts lower than where you started.
2 - That 12vdc converter page doesn't have really detailed specs, so a major question is whether it converts the total power or merely the voltage. For example, if you give it 48v at 3.54A, does it put out 12v at 14.16A (converting the 170 watts of power) or merely 12v at 3.54A (converting just the voltage)? If the latter, it sure wouldn't be a good conversion. ;-) Even if it does convert all the power and so converts the input to 12v / 14.16A, that output figure presumes a 100% conversion efficiency which the device won't have. You can probably figure on a 5-10% loss in this device.
3 - Last (and my apologies if it sounds like I'm piling on at this point), if the 12vdc converter does its job and puts out 12.0 volts, your batteries won't be charged. 12 volt batteries typically take up to 13.5+ volts to fully charge, and the batteries can't charge beyond the voltage that is being given to them. So, if the converter limits the output to 12.0 volts, the batteries will never charge beyond that point and will top off at about a 50-70% charge (rough number as it depends upon the particular model of batteries).
What might work a lot better is either to sell the panel on eBay (it should go for around $200-250) and pocket the difference or, perhaps, offer to swap someone the 48v panel for an equivalent wattage 12v panel. (Like me, for example. I have an extra 40-45watt 12v panel and am switching my whole setup over to 48v... ;-)
Craig