The most common models appear to be the TK, C1-13 and LX. Facit machines were made well, and they were used for longer while others may have been replaced. This means that, while well used, they were generally kept in better condition than is usual, and I have personally never come across one with any serious mechanical faults. On EBay, they usually sell for very consistent prices around £50, with a few outliers where they have been confirmed to be working.
For model identification, see here (http://www.johnwolff.id.au/calculators/Facit/Facit.htm).
For a highly detailed look at the manufacturing history of the company and differenced between models, albeit in German, see here (http://www.rechenautomat.de/Facit/index.php).
For a very comprehensive history and information on most of the models, see here (https://www.xnumber.com/xnumber/facit_history.htm).
The Wikipedia page here is an excellent resource, although heavily focussed on the company (https://en.wikipedia.org/wiki/Facit).
For a very detailed look at the demise of the company from a broader perspective, see here (http://disruptiveinnovation.se/wp-content/uploads/Facit-and-the-displacement-of-mechanical-calculators-Sandstr%C3%B6m.pdf).
http://www.rechenmaschinen-illustrated.com/pictures_Facit.htm
https://www.jaapsch.net/mechcalc/facit.htm