Hello and welcome to the forum.
Your obviously into your French oddities!.
.I personally don't have any Vel Satis or Avantime experience,but I have worked on quite a few XM's,and I've got to be honest,I would leave them well alone!.Reliability is poor,and they are an absolute nightmare to work on.
We do have a few Avay and Vel owners on here,and although the cars do suffer from poor build quality,they seem to be in love with their cars!.
failures are common on V6 models,and depending on which engine bank the
is on it can be an expensive job to replace them.
Neither car is to my own taste,but I can see the attraction.