Hi Linotype, I lived 27 years in France (on a permanent basis) & bought several cars, both new & secondhand. I didn't really find that used cars were excessively priced. It's probably true, however, that cars depreciate less quickly than in the UK, as the 'company car' market isn't so big.
Most of the normal advice applicable to buying used cars in the UK applies, but you need to be aware that French bureaucracy can be pretty complex. In order to transfer a registration to your name, you need a 'certificat de situation administrative' which you can get online if you have a copy of the 'Carte Grise'(Vehicle Registration Document). (See
https://immatriculation.ants.gouv.fr/Services-associes/Le-certificat-de-situation-administrative-non-gage-et-opposition) A decent car dealer knows how to cope with all that so it would probably make sense not to try to do it yourself unless a.) you speak reasonable French; & b.) are good at researching bureaucratic procedures on the Internet. You mention Road Tax - ages ago it was considered that French petrol prices were higher than those in the UK as they had no road tax. Then, for a number of years, local départments were supplementing their income by charging an annual fee, which varied according to départment & giving you an annual hexagon-shaped coloured sticker. This came to an end about 15 years ago, IIRC, & there is currently no road tax as such. Yes, there is a tax on insurance, but it's not that much & probably applies to other types of insurance as well. All the same, 'road tax' in some form could always come back, but is unlikely to ever be at UK levels. (The citizenry is too inclined to take to the streets for that.)
I don't know what your French is like, but a good site to consult is largus.fr. The Argus is a kind of equivalent to Glass' Guide in the UK & their site might be useful to you. Even if your French is mediocre, if you browse with Google Chrome it translates most of it for you. (The translations aren't perfect but help the beginner enormously.)
Good luck with it all. It can be fun as well as madly frustrating. In the end, I retired to Spain as we'd had enough & it's much cheaper over here.