The cheap Chinese gauges on eBay are very hit or miss. The pics, descriptions, and actual products often don't match up. If you're lucky, it will include a diagram explaining which wires are for which lights/gauges. If not, you'll have to do some testing to figure it out.
It's essentially just a matter of hooking up the right wires from your harness to the right wires on the gauge. They do make some digital speedos that are cable driven, and they make cable/electric adapters (
example). To convert to an electric speedo, you basically just have to install a sensor on the fork and magnet(s) on the wheel. Like a bike speedo, the sensor just tracks how frequently the magnet passes by and does a little math with your wheel size to determine your speed.
Koso is expensive because it's high quality, and they have a PnP kit for the 250 available. You're paying for a good product and support, rather than an undocumented, hopefully-what-you-want, questionable-quality cheap thing from China (as with most of the cheap stuff from China that you find on eBay).
https://www.ninjette.org/forums/showthread.php?t=189160 might have some helpful info.