One can certainy navigate with plastic sextants, the Davis Mark 15's and 25's being examples thereof. Watch out for "wandering index errors" though, as well as "instrument errors", which as I understand, are not easily determined, or certified as they are with metal sextants.
They, the plastic type, cost a whole lot less, and for a beginner or one who hasn't made up their mind, are something to consider. You can always upgrade later, and should you opt not to bother, you haven't put a lot of money
into what might becone an interesting paper weight.