Can you get a battery charger to the car for a day. OK I understand you will need to keep your eye on it as it is in the street. Does your car have the Pre-heater? That should warm things up, even in the boot (trunk).
Good - Fast - Cheap: Pick any two.
E23 735i; E32 735iL (X 2 - 1 Alpine White and 1 Glacier Blue); E34 535i; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E30 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i