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Speaking from someone who has gone through incremental steps. I second Frank's picks, with some added thoughts.

I started Davis M3, went to Davis plastic Mark 25, then to the Astra IIIb, and finally now own a C&P gotten on ebay in perfect shape from A British Petroleum Company trainer.

I found the plastic sextants with the drum a bit cranky on maintaining a constant index error. Some of it is you have to be careful how you hold them and grip them with my big hands. Some of it is if they get dirty they drag and distort sligtly (e.g twist with friction).  Some of it I improved it by purchasing  replacment mirror retainer clips, as they can get overstressed with excessive twiddling from inexperienced hands. The rack error is easy to go one way on, which is a good habit for doing lunars on any sextant.  That would be my chief reservation on the plastic sextants. If you get interested in Lunars, they really aren't up to it. Lunars are great practice and combine a lot of skills.

I also happen to prefer the split mirror as opposed to the to the full horizon mirror. I find the full mirror harder to read in low light levels or fuzzy conditions. The Davis Mark 15 & 25 have always been sold as far as I know only with full horizon mirrors.

Lunars drove me to purchase an Astra from Defender with my Christmas bonus years ago.  It was a great choice. Funny enough they accidently shipped me a whole horizon mirror, and rather then replace it, we nogotiated a good price and they sent me the split mirror add on in a little box, so I now have both. Interchangable in minutes with coin slotted screws.  A great thing if you decide you prefer one over the other.

The Astra has great accuracy (proved mine with lunars), is well supported by Celestaire with parts, they sell they 7x scope if Lunars become your thing ( I ultimately bought that too a year or so later).  I happen to like the filter selections better than the C & P in one way.  If you wanted to use the sun Semidiameter at sea to set index error, the Astra filters can change the colors between the direct and reflected image to make the difference super easy to see.

Ultimately if the budget can stand it, I find the Astra IIIb a workhorse that can do anything you could ever want to do.

I honestly stretched hard and purchased a new one, so I was sure it wasn't dropped or sold after abuse. Dreaming of lunars, this made some sense. On the other hand, my C & P was just plain luck on the used market.  It's like brand new.

MC