I'm not sure if you are aware of this but REW Reynolds is a top to the line British brand. They were usually made by Woodrup in Leeds. And sometimes painted by Ellis Briggs in Shipley West Yorkshire where I learned to build frames 50 years ago. This is a no brainer to buy it ASAP. I would greatly prefer this bike to one of the French production Mixtes.

On a mixte you don't have to worry about stand over height so what you want to do is get a stem that can place the handlebars at the same height as the saddle. Often Mixte frames were designed to use with upright handlebars. This often involves a stem that does not point down but rather up. Zoom makes adjustable angle stems that aren't that expensive and can fine tune the fit.