Installing a block heater is a fairly simple job - even if you are not sure which end of the wrench to use
In your area I would not even consider a block heater. I do not use heaters on either of my diesels and have never had a problem. Last year I was able to glow the 240D a couple of times and start her right up on Christmas morning.... it was -9F at 7am.
Keep a good battery in the car and glow her an extra time or two before starting on very cold mornings.... you should not have problems.
76 240D (W115) - 550K miles
78 300D (W123) - 200K+ miles