The issue with lower horsepower motors is that regenerative braking is not as effective. For the most part, regenerative braking can only absorb as much torque as it can put out, and anything above this has to fall back to the traditional brake system (or some other system).

I've been pricing out how to perform EV conversions as cheaply as possible. Battery capacity is by far the most costly section, and having a smaller horsepower motor isn't going to change how much power it draws (assuming driving habits stay the same). I've been basing my calculations off a 40HP motor and here are some preliminary numbers you might find interesting

On a typical car, a 200 mile radius will require around 44.4kWh of capacity (at around 45MPH, no stops). This is the capacity of 3483 3.4Ah 18650 cells, costing around $13.6k (you can bring that price down if you have some connections and more supply, but that's the cheapest rate I could find for a non-business). 100 mile range is also pretty reasonable. The motor and all other components cost less than $1.5k, man hours required for conversion is another main question, but hopefully that becomes more streamlined over time.