Jason,
Congrats on the new car.

On my 82' 240 the driver side door key does control all locks. would assume same for 81'.

Block heater on my car is next to the starter. if there is not one installed, you will see a 1 3/4 inch threaded plug with a 19mm (same as 3/4in.) hex.
If the heater is installed but missing plug-in cord you may just see the pins there were the cord should be plugged to.

The driver side fender well is where the glow plug relay is mounted on my car. Check the fusable link (50 amp I think) located under the relay cover. You can find your relay by following the wire back from the glow plugs. You can by pass this relay I understand by bringing 12volts from the battery. and disconnect after car is running. DO a search for more info on that.

Starting problems can be many things, however, if the car started fine in 40 deg. weather I would start with thorough check over glow plugs & relay.

good luck
dave