Wow! You got your work cut out!
The fact that your 110 throws breakers and you have no 12 V working, I would check the converter first. Sounds like it went bad. The first thing I would do is try a known good 12 volt
battery and see if anything in the 12 Volt
system works. If it does that may point you in the direction of the converter. Next, I would shut all breakers off, then hook to 120, and turn the breakers on, one at a time, till you can isolate the problem.
As far as plumbing goes, you will be better off to start from scratch, rather than patch. The plumbing system is relatively simple in my opinion, but I am the son of a plumber.
If you go with PEX, you will have to buy a crimping tool for around $100, unless you know someone you can borrow one from. If it has the original copper, and someone did not winterize it properly, the copper expands and standard copper fittings will not fit the pipe due to the expansion.
I don't have any experience with leaking windows, but plenty of folk here have, just wait a few, someone will chime in.