Screw on freewheel on a fixed hub thread?

I am planning to build a single speed with Mavic ellipse, many places on the internet say the rear wheel is flip-flop but many confirm that they are fixed on both side, so I am looking for freewheel options for it.

I am sure there are people out there riding them with single speed, can you recommend a good freewheel that fits and not too expensive (17t or 18t preferred)? 
I also came across some posts saying that this one [http://bricklanebikes.co.uk/posts/393](http://bricklanebikes.co.uk/posts/393) from Bricklane bikes will fit, but they look a bit expensive at around 50 quid. 
Many thanks.