Need guidance for size of LED array...

[andru24]

So I am having RapidLED hook me up with an 18 LED dimmable DIY kit for my 20 Long (display area 24x12x12. First off, is this enough LEDs? Second how wide/long should the array be? I am thinking the 5.886" profile heatsink from heatinkusa.com @ 12"-14". Does that seem right? And what angle optics should I get? The fixture will prob be 6" from the surface.

 

Thanks in advance...