Can't advise for #1, but for #2 I may be able to offer some minor help. If there's room for 4 15gal kegs, you can definitely get a few cornies in there. Take a keg and trace a bunch of circles around the base on some cardboard, cut them out, and see how many you can fit within the interior footprint (similar to the sizing your chest freezer sticky). At that point, converting to cornies is pretty easy. Depending on how its been treated and how old it is, you may want to consider changing out all the lines anyways. Once you do, you could either include barbed corny QDs, or use MFL QDs so you could switch back and forth between sanke and corny connections if the mood ever struck you. If you wanted to stick with 4 taps, that would be the easiest. If you wanted to increase the number of taps, you could either add another two-tap tower or switch out the two current 2-taps for two 3-tap towers. That would save the hassle of cutting a new hole, but then you'd have to buy two towers instead of one. If you lager, you could also test whether you could fit 4 cornies and a lagering carboy in there, making the use of all the space but saving the expense of upgrading the taps for the time being. The extra space could also be used for lagering or aging in cornies, even if you don't have enough taps to cover all the kegs. If you added a splitter to one of the CO2 lines, you could even have a keg carbonating while its untapped so it was ready to go once you've kicked another keg.