Get the 300d. I think you will probably be able to work through it with out too much trouble. These cars are not all that complicated. If you are somewhat mechanically inclined and like a project it will be a positive experience. Having the right tools is important. All the running gear should bolt up ok. I put an 85 turbo motor in my 80 wagon with a 240 d 4 speed manual and a 3.07 rear end as my first benz project. It only took me about a week all said and told. The hardest part was finding all the right parts with out going over budget.

I spent: 750 motor, 100 trany, 50 clutch, 45 flywheel balancing, 60 pedal assembelly + master cylinder, 60 new slave cylinder, 130 drive shaft cut + balance, 125 rear end (worth changing), about 150 odds and ends.

If yopu have a whole parts car with everything you need it will be much cheaper and easier for you. You also have a prety good suport network on line. Take your time, ask qustions.

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1980 300TD with 1985 300TD Turbo motor and 240D 4 speed.