Well he said 50kph(30mph) not 60kph(35mph). (There IS a slight difference)..
I'm sure the bike will do 50kph but not at 36v 15a (500w)..
On that same bike with just me and my 36v 20a (720w) I can get up to 23mph (40kph).
In order to get it to 50kph(30mph) it took me a 36v 30a controller (~1100w), BUT there is NO WAY, to keep that speed for any length of time, unless you have the WH and the C-Rate to do just that and that's not cheap. Unless you get stuff for 1/2 price..
36v 30a is gonna take you about 1200w (realistically) to keep that speed to ride for an hour (30m/50km) that's 1200wh that's a 36v 35ah battery at the very least..
So Let's calculate 36v 35Ah / 36v 30A controller / motor kit capabilities to match that ability to keep that speed/time wanted for just 30mph/50kph for 30m/50km..
Figure JUST for the motor kit a minimum of $400 and that's being way generous.
(Probably will only last 6 months, but we didn't specify how long it has to last...)
The battery is gonna cost you $600 in LiPo. So we are at $1000, BUT we haven't figured in the power supply, the balance charger.. Figure $200(low ball price) for decent ones of those, we are at $1200 (using LiPo) and I didn't add some kind of monitoring during ride, but I digress..
Using lifepo4 (with charger and BMS), I found a 36v 35Ah on eBay for $650, so again close to $1000.
Add a $100 for bike, torque arms, watt meter..
Now still about $1150 for 30mph for 30m (50kph/50km)...
Try going 35m/35mph and you're looking at another $100-$200 in battery, $100-200 in motor, $100 more for a better bicycle.
Now we're at $1600 and we didn't even include shipping.
(Just in case, you just happen to live next to a place that can pick these things up locally..)
Now if you went the "retail" route (Here in USA), you're looking at double or triple those prices..