Sears installed my first set of windows and several other contractors have stated that the installation is great and that the window itself looks great. Had some pretty damaged original 1911 windows in a few places. I think that for my house, it DEFINITELY improved the value and I know it's more efficient, but it depends on what kind of house you have. Mine is very old and has single pane glass, so almost anything would be better.
The second set of windows, we just measured the windows, special ordered when HD had a major sale and found some dude to install them with us. Worked well & was much cheaper but they don't get us the same compliments that the professional installations did, mostly because we didn't redo the exterior plastic trim around it. Since you need sills replaced and presumably have wood rot, I'd call a factory install. Every place that came to me made me spend about 1.5 hours with them, but of course, we had a lot of windows to measure and a lot of house to see. How big are your windows?
The electrical work I had was just a union electrician. Spent about $800 on a new circuit box and about $800 on the new electrical line to the garage, but we paid more because they had to drill through cinderblocks to get into the garage. And when a union electrician does it, you know you're following city code. Good luck, Eremite.