new thermostat wire might have to be pulled from the t-stat to the air handler, and then, of course, you need a condenser.
I'm an hvac professional. Maybe some of my hvac hubs would be useful to you.
As far as T stats go. . . .you plan to find a subbase for your old mercury bulb t-stat? Why? Get a digital one that can be programmed for whatever application you have.
I prefer the Honeywell Vision Pro 8000-you'll have a hard time finding a new sub-base for an old t-stat. It's only a matter of time before you'll have to replace that thing anyway.