That's a really cool little heat sink you found. I'd try a 20W on it, though, I think a 50W might overheat. Maybe a 30W could work, but I'd wanna try the 20W'er first just to get an idea of what the thing's capable of. Also, you'd wanna ask what that fan is rated at in terms of Volts and mA so you can locate a suitable power supply. I'm guessing it's rated for 12VDC x ~90mA. Check the specs on a 20-30W LED driver, the motor would burn up within minutes, probably less.
And for giggles, run an eBay search for "Arctic Cooling Alpine." That company makes some really nice stuff that tends to go for very reasonable prices. They also rate their CPU coolers for how many Watts they can dissipate which makes things really easy for us. The little Alpine 64 can cool 70W and should be $15-$20 shipped. Better safe than sorry, imho.
And hey, if you save old chargers, wall warts, cordless phone power supplies, things of this nature I'd check your stash and see if you have anything in the 12VDC x <100mA range. That, or take a walk down the electronics section inside a second hand shop, maybe hit a few garage sales for random goodies. If you can't locate a PSU that suits your needs for free you should at least be able to find one for a dollar or two at one of these other locations. If push comes to shove you can find one on eBay, but you'll pay a premium.
I can make the 50w led cheap. The heatsink is $5, 50w led chip $12.89 and the led driver $11.29. Total $29.39. That equates to 1.7w/$. That should give me a total of around 75w.