I haven't heard of the EC freewheel you bought, but it looks ok and price is good, so it's worth a shot.

I've personally always gone with the Shimano MX30, although it does have some downsides. Out of the box, it's fantastic, but it does become a little noisy somewhat quickly. It develops a clicking sound with every revolution at certain cadences. The one I'm currently using has even started to slip (the pawls skip) when I'm starting from a stop and putting a lot of torque into the pedals...no bueno.

On the upside, the MX30 is fully serviceable if you're somewhat handy. I have several that I switch in and out between seasons, so no current need to ever buy a new one.

As far as your gear ratio goes, I'd recommend taking a road bike out for a spin (if you have one) to find a ratio that you feels works best for your needs. Once you've found one that works, use Sheldon Brown's gain ratio calculator to see what freewheel size that translates into for your size chainring (because a road bike will almost certainly have a different size chainring than your single speed). Hope that helps.

Sheldon Brown's Bicycle Gear Calculator