R & R Model Engineering
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These would be starting points for your internals and knowledge. You can build your own or buy pre-designed WTC. If I may offer a couple of suggestions, one, pick an easy sub like the Revell Skipjack or Bronco Type XXIII
Both of those have plenty of room to work in. Second suggestion, do not go crazy with all the fluff like torpedoes, working scopes, and such. Work to get the sub running and working well. Then you can work on another sub and add the fluff since you now have the experience. A statement I was told was crawl, walk, run.