You're thinking in the right direction here. Work on the stuff that has to be done first (plumbing, electrical structural, etc.). As usual, everything with a home ends up costing more than originally planned. This is why contractors usually double their estimated cost up front, because they've been there and done that, and they can't afford to run out of money a week before completion of a project. Don't be afraid to wait if you can't afford projects yet, just make sure when you actually do them they end up like you wanted. A house is not a money pit (usually) if things are done right the first time through.
As you are observing, stone walls have a huge amount of thermal mass. This does not necessarily mean that they have high r-value, though. I believe concrete block walls have an r-value close to 1. The airspace and gypsum probably have close to the same r-value, but they have very little mass. The result is that the wall gives up and takes in lots of heat, but it takes a long time to feel the change indoors. This effect will catch up with you in the winter, right around the holiday season.
The quickest and easiest way to insulate your block walls is from within. Injecting foam into the hollow cavities inside the wall will increase the r-value to somewhere above 10, and you don't have to rip your siding off to do it. This will give you the most bang for the buck, especially if you DIY. It also will seal the block wall, killing infiltration issues. The stuff goes in like shaving cream and sets in 2 minutes. Plug up the holes you drilled and it's done.
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