Sounds like you will be pulling power eventually to make a switch over. Somewhere in there, a code inspector will probably become involved. Most power companies won't restore power to a structure till a permit has been taken out and a permit issued and signed off. Where I'm going with this is that you probably need to consult the building inspector if they will have to "buy off" on this. While the NEC is pretty open or vague on things like panel location, many localities have specialized rules of their own.
Also, while the residence may be old and small and not need more than 100 amps service, many localities won't issue permits on anything smaller than 150 or 200 amps on a residence.
If the panel is to contain the main disconnect it must be accessable by an average person standing on the floor. Code calls for 6-1/2ft headroom, 30 in wide and 36 inches deep clear in front of a panel, but makes exceptions for headroom for 200 amp and less in existing dwellings and some access exceptions. It can be located behind an open door if the door can closed and the clearance requirements met. You also might consult an electrictian and see what the local needs are. Hate to see someone get all done and find out its not acceptable due to local requirements.
Charles
Charles
