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My recommendation would be to spring for a new or used Astra IIIB with the SPLIT horizon mirror and not the WHOLE horizon mirror. If you're just starting out the whole horizon mirror will make shooting sun sights fairly easy compared to using a split mirror. But it(whole horizon mirror) is almost totally worthless in taking cuts of stars or planets especially from a small, bobbing and rolling vessel.

A new or good used Astra IIIB gives you good reliability, practical accuracy and service for the money spent for obtaining it. If all you can find before starting your voyage is a whole horizon mirror Astra IIIB get it and use it with confidence. It will just be harder to take cuts of stars because ½ the light from the horizon is reflected away from the sight taker.

Stay away from plastic sextants is my advice. Not meaning to degrade plastic sextants but because of all the special procedures needed to use one within practical accuracy parameters(especially a novice sight taker)you will have more enjoyment taking the cuts with the better frame and scopes of the Astra than on the best of the plastic sextants(Davis # 25).