1W or other "power" LEDs require a constant current driver, as they are prone to current run away and hence hot and painful death.
Power LEDs are also far more costly than standard LEDs so I wouldn't expect you to get much change from £119 for 119 LEDs! Add a PCB, heatsink, constant current driver, AC/DC converter and housing I doubt you'd build it for less than what the seller is asking for them!
I think grow lights are also pulsed, hence the frequency on the listing, you'd need a control circuit for that too.
You could certainly build a lower spec version for less but to get the same result as you would from that product, the cheapest method - in my opinion - would be to buy that product!