Rear. After pinching brakes and group off a 29er I got restless having most of a bike but no funds to build it again. Velosolo cog was £12 used off eBay + fixeh chain I had hanging around.
135mm fixeh options were pretty limited; surly hub was around 100+, then it got really spendy
Depending on whether you want to run a brake (disc brake anyway) it may work out cheaper to grab a used wheel and use a bolt on cog.
I did it recently and chainline was bang on