Nil. Visa straightforward as long as you have a plan to return and money and air tickets and Google translate can translate everything when you are there
2 weeks for France, after going to the TLS office in BKK for fingerprints. , Who could only be booked in 7 days time. Have all the docs and financial support letters(if needed) is all easy
There's a few companies that can help with the simple stuff like indicator lenses , Speedo etc mainly for US imports so check them out. But potentially the biggest issue will be type approval and emissions compliance for the year of vehicle as Thailand way behind Europe. May (will) need to get it through a single vehicle approval route.
Seems like a lot of effort and cost when used car values in UK are low.