I want to insulate an older store attic.

There is no insulation there now. I can see the ceiling joists which is 2x4's. This building is old and has wires running across the ceiling joists.But it has been remodeled to make two stores from one building. Above this it has plaster and latt boards, then I'm guessing the roof joists. The plaster is falling from the latts in places but I think from age and not a leaking roof. I don't get any water on the interior ceiling.

Also the furnace is in this area at one end.

Can I use Blown-in or should I use fiberglass batts?

I didn't notice if the wiring was old fabric covered or newer sheathed electrical cable wire.

How close can I get to the furnace and not cause problems? Would I have to build a dam around the furnace? At what distance from the furnace? I still have to change the filter at the furnace.

Any suggestions will be greatly appreciated.