I've heard that a fuse panel being present can be a problem with bank financing, so that might give you some leverage or kill your financing. 

Updated electrical panel and furnace.

So really the only connection point will be where you punch through to allow access between the two buildings.Depending upon the electrical service of the old house, you may need a new, or separate sub panel for the new structure.

If this is all exterior lighting, it would probably be most cost efficient to surface mount the panel and your home runs.

Power - need to assure you have capacity in your panel to add the load for this additional space.

I've been residing with cement board such as Hardi panel and Hardi plank lap siding.

I ripped out all electric, moved the panel from basement to the apt, moved the kitchen to a new spot and moved the bath to the old kitchen.

This Bill (AB 693) allowing the state to offer incentives for solar panel installation on low income apartment buildings is interesting:   https://leginfo.legislature.ca.gov/faces/billTextC... 

Most include windows that are too small for the average-size person to escape from and wood paneling situated too close to the furnace.

That said, there's about $5-6k in roofing, both bathrooms, both floor kitchen appliances that need to be replaced, electric panel needs to be converted from fuse box to circuit breakers and the upstairs furnace seems to be on its last leg.I don't think I have enough to do all the rehab at once, and the roof seems to be holding for now.