Reply to Buying 'broken' Kontrol S4 to Repair, difficult?

I'm seeing some relatively cheap S4s and other controllers and whatnot go for pretty cheap on eBay with the caveat of being broken or not known to be working.

Currently looking at an S4 that doesn't have its lights turn on.

Has anyone done their own hardware repair such as soldering and whatnot and can tell me what I'm getting myself into? I'm relatively capable with a soldering iron so I'm not worried about that. More worried about finding out parts cost way more than just buying used controllers that do work.

I realize this is hard without knowing what part is actual broken, but any insight would be helpful.

Thanks