I can tell you what we did...
talk to Everest - laugh at their prices (£60k), wait for a discount (£40k), laugh again, wait for another discount and consider it (£25k).
talk to a builder - realise he is still £10k cheaper than Everest's best effort.
We got our frames made up through the builder at a local woodwork place. I met the carpenter/joiner and discussed options. I specified treated (so even if the paint fails they won't rot) hardwood that would be supplied manufactured and painted by them.
They measured up, built the frames, dismantled the frames, sprayed them (2k paint so I'm expecting 10years before a recoat), rebuilt the frames and fitted them. Builder then replastered etc.
We specified the glass ourselves from Pilkington. There is a vast amount of info available on their websites and on energy efficiency sites. WE specified K-glass with an extra coating to increase thermal efficiency. We looked at argon filling but it was £££ and can be unreliable. We went with warm edge spacer bars over standard metal.
There was a bit of leg work and plenty of reading involved. But we were spending about £1k per window and we have 15 of them (sash).
Our builder was very good and gave us the confidence and advice we needed to guide us through the options. our victorian house isn't standard so off the shelf units wouldn't have fitted properly so each window was made for each space. We used a lot of the info available from the likes of Everest to help us spec our windows. We looked at what they offered and basically matched the better stuff.
We never considered UPVC but we did look at restoration which was typically 2-300 per window and included replacing rotten wood, sanding and painting so they all opened and closed freely and fitting proper hidden draught exclusion.
Our house is much quieter now and significantly warmer.