Yeah same here. Its not worth the $ to do unless you really really love it.
It prob cost me about $1400 for the motor, controller, battery. So your normal Baja mini goes for $400-500, adding these components to resell it, you would never recoup your costs.
Here are the major purchases for the conversion:
Motor: motenergy/mars 4201
Controller: Kelly KLS7222H,24V-72V,220A SineWave Controller
Battery: 52v Samsung INR18650 25r 20ah 50a/80a max - Triangle battery (for ebike)
Motor Sprocket 12t - Rear Sprocket 72t
Its possible to go cheap and use a chinese controller, I would not recommend using SLA batteries as they are Heavy. Lithuim 18650's packs with a balancer on them are easy for plug and play charging. Expensive though. RC Lipo's are cheap, but you have to know what your doing or fire may happen!