Their prices are pretty much compiled using an algorithm,not real world values;I ignore that site.

Better to compare prices on eBay and whatever local sites you have. As to that bike,from the pics it seems to be in decent shape. The chain may need replacement,or just a good cleaning. The dyno may just have a loose wire somewhere,and replacements aren't hard or expensive to source. Also note,Brompton design hasn't changed much in a long time,so you can always upgrade to the newer parts.