Merc SL... to register or not to register?

Hi. So the back story is I live in the UK and I have a friend that I regularly visit (like every 2 to 3 weeks for a couple of days each time) in France - that is when Ryanair haven’t cancelled all my flights…

I’d like to buy a '93 Merc SL 300 and leave it in France for when I come over. It is LHD and originally registered in Germany but is now on UK plates. I am aware of the 6 month rule, so bearing that in mind - what is the likely cost of putting it on Fr plates in my friend’s name? I know that cars over 10 years old get a 50 off tax Regionale so I can probably work that out, but I’m confused by the pollution tax… which will no doubt be horrendous. But - the bit that confuses me is that it reads that it only applies to vehicles registered after 2004 (is that new reg or reg in France?).

Anybody got any advice as to what I should do? Bring the car back every 6 months or reg in my friend’s name?