Alan - I think you will like your 300D in Minnesota. I ran three of them over the years - my last one, an 83 was certainly the best. My experience:
Rust proofing: From around 83 on, the factory did an inside and outside rustproofing. The inside is an amber colored semi-solidified coating, the external is a heavy black undercoating. You could do nothing and be fine in Minnesota for years - especially if you get into the habit of washing the car once a week in winter months. Flushing the underside and redoing the undercoating cannot hurt at all.
Block Heater and winter tips: The MB block heater is a low-wattage type (I'm estimating 500 watt) that I plugged in every night when I pulled in and unplugged when driving out in the morning. I also plugged in at work during the day when it would sit for 4 or more hours. For the occasional overnight on the road when I didn't have access to a plug in, I carried two keys and let it idle all night. I estimated it took 1 1/2 gal of fuel to idle for 8 hours - a small cost for a great comfort and wow, how nice to come out to a warm car in the morning. These diesels 'cool down' when idling, sort of like going to sleep. Leaving it unplugged for short trips during the day is no problem. The colder it is the shorter the time that it will comfortably re-start with no plugging in. i.e. I could drive to a store when it's 0 degrees F and have it pop right off after 1 hour. If you get stuck with no plug in and the engine got really cold, double glowing the glow plugs definately helps starting.
Oil: My experience, these older diesels don't seem to respond to synthetic oil like newer (TDI) diesels or gas engines. I purchased my 83 with 212k miles where the previous owner used straight 40 wt. diesel oil in the summer, straight 10W wt. diesel oil in the winter and the owner reported the car used zero oil. I switched to using 15W-40 Rotella T dino oil in the summer, 10W-30 Rotella T dino oil in the winter. (10W-30 Rotella T is available by the case at Hallman Oil in St. Paul) When I sold the car with 320k miles, it still used zero oil between my 2500 mile changes. If you keep the car plugged in when the ambient temp. drops below 32 degrees F or so, you could run 15W-40 year round. These engines love that stuff and have heavy duty starting systems that don't seem to know the difference. I tried a synthetic Delvac 1 oil one time and could tell no difference in this vehicle. I settled in to using the lower cost Rotella T dino oil and doing more frequent change intervals.
Fuel: In the past, I ran any winter blended diesel that all self serve stations stock in winter months. If the ambient temp was heading for 0 degrees F and below for an extended time, I would head for a straight number 1 pump and put in a half tank or more of that to buy me insurance against gelling. Also, it helps to start each winter season with fresh fuel finters. Dirty filters accelerate plugging when fuel gels. I would also add a diesel anti-gel additive (like Power Service - available at all truck stops) to every other fill or so during winter months. Nowdays, a number of Minnesota stations stock 'Arctic Diesel' which is a high cetane, winter specific diesel. Check
www.premiumdiesel.com for a listing of stations that stock Arctic. I use this exclusively now in the winter months in my VW Beetle TDI diesel.
One last recommendation: Visit and get to know Warner Huber in Golden Valley. He owns Huber Imported Autos and I believe is one of the most knowledgeable technicians around for 123 diesels. He has serviced all my MBs for about 8 years now and he knows how to keep the older MBs running without breaking the bank!
Regards,
Wayne