Hi, a friend of mine want to sell me a Brompton electric c line from 2022 for a really good price. Unfortunately it is a high version and I will much prefer to get a medium size.
Does anyone know where I could buy a medium stem in steal Brompton or after market of a good quality.
Is the upgrade of the stem difficult, knowing I did fix a lot of bikes in the past. Will I need to shorten some cables too?

It'd be simpler to just put a lower bar on it. Nitto and others make low-rise narrow bars for the fixie market, they should work on your bike without interfering with the fold. A benefit here is you'll have the original configuration available to put back to stock if you decide to sell your bike later.

I can recommend Joseph Kuosac handlebars which are mid rise or low rise. Should not interfere with fold. They have 12 degrees back sweep which is much more comfortable for my wrists and also are wider than standard which provides better handling in my experience.

Ideally, with another stem or lower handlebar, all cables should be shorter.

Brompton has different cables kit with different length for the different stem.