I would suggest a W126. Look for a 84 or 85 300SD. For a car without airbags they are about as safe as it gets, AC works decent if maintained properly and these cars are fairly abundant.

Learn to work on it yourself. Start by changing the oil, filters, fuel filters...
You have a huge amount of support here on this forum while you're going thru the learning process. Don't even try the 'veggie thing' until you know the car well and understand the fuel system.
Soon you will be adjusting the valves, changing out ball joints... all the other easy stuff that most of us do here. That link at the top is a great place to buy parts btw.
When you get to that level your 'new' old MB diesel can become your daily driver. Until then don't rely on it being ready to go when you need it.

When looking for an old Benz drive several to make sure you know how they should "feel" before making the purchase. Get familair with terms like "Flaring" and I'm sure you will find it in a car or two that you test drive. Don't get in any hurry. There are usually a bunch out there for sale. Be patient, learn as you look.
A pre buy inspection by a mech very familiar with these cars would be a good idea.


So if we haven't scared you off, welcome to the forum and good luck.