Tilda, with replacement windows, doors, kitchens etc, Rule 1 is to stay away from salesmen. There are plenty of fabricators who make up uPVC windows etc to size, just as a joiner would in timber. Many carpenters/joiners now specialise in ordering/fitting uPVC components.
I'm not saying that a firm couldn't measure, make, and fit as well. Of course they can, but then you come across the minefield if you do not have a recommendation.
I often repeat on here ..... ask around, particularly at a local builder's merchants for names. A good tradesman will help you in the same way that any other building components are bought and fitted. Try and get that personal touch. It does work. Also, pub landlords are one of the best sources of local "who's who".
uPVC, Alum etc ........ just a matter of personal taste. It's not particularly hi-tech. Quality is much improved over recent years. My pet hate with uPVC is the hideous "brilliant white". I usually paint them. My personal preference though is powder-coated aluminium. Plenty of colours, and a more pleasing modern look.
Local joiner for bespoke interior doors too.................. prepare for relatively high cost though.
Design, materials etc can be found later. Find your tradesman first.