A rear hub motor conversion can get pretty involved. Most require 135mm (or more) dropout spacing. If you're converting a modern MTB or hybrid, you're probably ok, but road bikes or older MTBs can be a problem. Most rear hubs are set up for a freewheel, which will get you 7-speeds at most. I've seen a couple with freehubs that will take an 8-speed cassette, but the choices are much more limited.
A front conversion is usually cheaper and easier. You have a huge range of options and a much simpler installation. Your dropouts will be fine as long as you put a torque arm on both sides (which I recommend for safety even on low power motors).
Note that much of the cost of an ebike conversion is the battery, so "low cost" conversions are going to be in the range of $600 - $1000 for a 250W motor laced into a rim, 24V LiFEPO4 battery, battery charger, motor control electronics, throttle, e-brakes, and torque arms. If you want more power or the ability to cruise at 20+ MPH without pedaling, you're looking at $1000+. If you don't mind hauling around the equivalent of a boat anchor and replacing batteries every year, you could go with sealed lead-acid batteries and save at least $100.