If you want 250 watts or less for a bit of assistance with pedaling and ~20 miles of range, a conversion kit is fine.
But if you have to deal with serious hills or want to use throttle-only, 250W won't be enough. IMO 750W is the bare minimum for that, and it requires a bigger motor, bigger battery, and disc brakes.* That costs a lot more and it's easier (and probably cheaper) to just buy the whole bike than convert what you have.
*Disc brakes are essential for higher-powered e-bikes because such bikes move faster and are much heavier than regular bikes. They need more serious stopping power than rim brakes can provide.
Currently 250W and 15.5mph are the current legal limits in the UK. If want buy bigger and take it on the road without registering it as a motorbike, getting an MOT and insurance or paying your road tax, then you may find the rossers feeling your collar.
I ride with a friend who has gone MTB electric. I leave him behind on the flat but he gets it back on the hills. He's fine for well in excess of 40 miles. Range is mainly an issue of battery size not the wattage.
If you want to use throttle only, then you're no longer riding a pedal-ec and are riding an electric motorcycle (and will consequently need the appropriate licence, insurance and vehicle type approval).
In the US if the bike is powered by a 750 watt or smaller motor and travels at 20 mph or less, it's still an electric bike -- not a motorcycle -- even when powered solely by the battery.
> 250W won't be enough. IMO 750W is the bare minimum
Those numbers seem huge. I can only put out about 150W sustained, so a 250W motor boost on top of that sounds like a lot. That would have me at 400W combined which is nearly pro-rider territory.
Pro riders are riding 17-lb bikes. An electric bike with a big motor and battery is closer to 60 lbs. Electric bikes are governed by something similar to the rocket equation.
But if you have to deal with serious hills or want to use throttle-only, 250W won't be enough. IMO 750W is the bare minimum for that, and it requires a bigger motor, bigger battery, and disc brakes.* That costs a lot more and it's easier (and probably cheaper) to just buy the whole bike than convert what you have.
*Disc brakes are essential for higher-powered e-bikes because such bikes move faster and are much heavier than regular bikes. They need more serious stopping power than rim brakes can provide.