I got a battery powered RC 4017 engine for about $30 on ebay a few months ago. It was in great condition except someone left the battery in too long at some point or something, so one of the connectors needed to be sanded shiny again. That seems to be a common problem on 2nd hand playmobil battery powered trains. It runs very well now after I gave it a quick rub with fine grit sand paper on the discolored connector when I first got it. It was only missing the battery(the old nicad batteries are just about useless anyways), and the controller, but I got a new controller for $13 with free shipping on ebay recently and a non-playmobil but compatible 27 MHz AM quartz set for $5 here:
https://www.ebay.com/itm/161797279816 (they send a random frequency pair, so go by the text, not the picture; I ended up with a #3 red/brown pair). I had a spare battery holder which now holds eneloop nimhs. I think you can still get the AA battery holders from playmobil UK for under 10 pounds. I've seen the 5258 on ebay new, 'buy it now' with free shipping, for $150 a few months ago, only the box was damaged. You could look around and wait for a good price rather than just buying what's available at this very moment, because decent prices pop up semi frequently. But beware of missing bits. If you get a deal on a skeleton of a train, you might end up paying more for the replacement bits than you did on the train. A particular roof is usually only offered for about $70+, while other kinds are $10-$20, and chimneys can cost a bit much as well, not to mention that prices for a lot of other discontinued parts can add up.
-bbbb