A sextant would obviously have to be accurate, but I don't know what level of precision is needed for ocean navigation, or how precise sextants can actually get. But I imagine there's a point of diminishing return spending more money to get more precision. For example, I have a watch (analog now, with my aging eyes!) that has a second hand. That's all the precision I need. And I don't even look at the second hand during daily chores! A watch with a 1/10 second hand would be 10x as precise, but probably no more accurate, and certainly no more useful than my cheaper watch.

For my uses, which are to simply learn the concepts and techniques of Cel Nav but not actually use it for real navigation, I'm sure I could get by with an inexpensive sextant. A very inexpensive one. But I'm also looking forward to using my friends, under his guiding hand, so I can learn the finer points of sextant use like the drum and vernier scale that will give you higher precision.

Time will tell if I can get my fill of the finer points with my friends sextant and then opt for something like the Mark 3. Or if I'll want to step up (relatively speaking) and get a Mark 15. Or if I get enthralled by the fine mechanics and end up buying a metal one, which would mostly be for display and admiring in my home.

But I'm just at the very beginning of all this. I've read the theory parts of Blewitt's book "Celestial Navigation For Yachtsmen" and I've gotten a few chapters into Burch's book "Celestial Navigation".