it's been a while since I was selling acoustics (new, used, and vintage) so I could be off.
I would guess around $4000 if the Martin was in excellent shape, original finish all around, didn't need a neck reset, had sufficient saddle for action adjustment, and good neck relief (remember no truss rods). Maybe between $3000-3500 for a guitar with good refin, but again no significant issues a good luthier couldn't handle (sounds like the Martin you have been offered). I would then say below $3000, possibly under $2500 for a guitar that had neck issues (which including proper repair work, would bring the total above $3000 again) and finish issues, and/or body issues (patches, multiple repaired cracks, etc.). WHen you start to factor in repairs, I would think the total would be $3000+
Again, I've been out of the scene for 3 years now, but back then a similar guitar would probably be $500-$800 cheaper.
Two things to consider: the 0-18 isn't a real popular model, obviously a D-18 would command more money.
Second, considering your in France, I would think coming across a vintage Martin or LP Jr. would be harder than in the US.
Ultimately, if the Martin sings for you (0-18's have a very specific voice, and don't work well for all types of music) and you can let go of the Jr., I would say go for it.
A refin 0-18 is a $1500 guitar, tops. A top refin is the worst part -- because this is what runs the highest risk of being thinned.
A nice '59 Jr. is now going for $10k plus.
In exchange for a Jr., you could easily get a shade top 1930s 0-18, with room to spare. Or, a '50s/'60s 000-28 or D-28. Or... lots of things more valuable than an 0-18.