Fitting a kit to a decent mountain bike can be a little tricky unless you are good at making minor mechanical mods. If your bike has rear suspension, the first thing to think about is how and where you will fit the battery. The kits either give you a bag to fix to your rack or a complete rack with the battery that won't fit to a rear suspension bike. Next thing to think about is your gearchangers. If you have twist type or quickshifters, you'll have difficulty fitting the throttle and handlebar switches, Thirdly, there are thin and thick pedal sensors. The thick type won't fit if you have a tripple chainwheel. All these things can be overcome if you are good with tools, but require a lot of head scratching and more money.
I had the same idea as you so I've done all this, and I also bought a cheap ready-made e-bike for about the same cost as a kit, so my recommendation is that you get a cheap ready made one. You'll probably find that it's ok, but if you don't like it, you can figure out how to swap the electric bits onto your other bike. Kits with batteries start at about £400. Ready-made bikes start from about £500 and some of them are basically cheap bikes with a kit already fitted. You have to ask whether you need all that hassle for the sake of £100 difference. Have a look on Ebay for cheap e-bikes. Then, when you choose one, post a link here before you buy it and I'm sure everyone here will be able to point out the good and bad points.
