Why does a young man want an electric bike? If he is healthy then he is probably capable of going generally faster on a standard bike than any legal electric one. Did you know that, legally, the cut of speed for assistance is 25kph (15.5 mph)? Also the wattage rating limit is 250W. There are many kits that are faster and bigger but not legal to use anywhere except private land to which the public do not have access. Hope you have a big estate !
Also the bigger the motor and the faster you go the less the battery range, significantly less.
The hellcat looks like a nice bike and I guess it would take a bottom bracket motor or rear wheel hub quite nicely. With a bottle battery on the down tube the balance would be good.
I have a 250W hub motor on my bike and its hill climbing ability is outstanding. I'm in my 60's and, believe me, that motor does most of the work up my local welsh hills. So, if he wants it to help uphill a legal kit, wisely chosen, will do the trick. If it's speed he wants then it is going to be on the shady side.
My kit is an Ezee from at Cyclezee who posted earlier.