Your ”spare” outboard will not be reliable if you don’t run and maintain it regularly. Your diesel will be more reliable if you run and maintain it regularly, and more reliable than an OB if they both sit idle (diesel fuel is far more stable and far less volatile).My Yanmar 3GM30 has 4000 hours ( 20 years old) on it. I'm thinking with an outboard too I'd have a lot of security, both in terms of propulsion and steering, plus the outboard could double as a dingy motor if I ever get a dingy solid enough for it. Any thoughts? Thanks!
Save your money or put it into maintaining your diesel. 4k hours is no big deal - it can last 10k+ hours if maintained properly. After 20 years you may need to change some parts, especially rubber hoses and seals, or corroded parts.
If your engine dies, sail. If you can’t sail, use your anchor and call for assistance".. If you can’t sail into your slip, sail into the harbor then call for docking assistance when near. A Towboat US or Seatow membership is pretty cheap.