Depending on siding, a piece or two can be removed and blown in insulation put into walls by drilling walls. Blown in can help with attic, even if it is poorly accessible. Used commercial carpet tile are inexpensive on Craigslist and can be pulled and washed or easily replaced when bad. Depending on slope, cut 2x4 on angle and put subfloor in, or use concrete or floor leveler to level floor. If solid concrete, glue down osb/plywood and lay flooring over it. It will insulate some from the concrete.
A wood insert or propane would definately help a lot. It is a lot cheaper than electric. Consider a propane wall heater if need be, or a wall mount furnace. Heavy curtains over windows helps as well. The door may be better off just replacing, but there should be a weather stripping to fix almost anything.