Hey, I've searched and read up, but I'm still having trouble figuring out how this rear spacing works.

I bought my bike off of craigslist from a guy that used to own a bike shop and is now selling his old frames as singlespeeds because "they sell quicker". The chainline is straight and it rides great. I'm looking to convert it to a fixed gear bike so I'm gonna get a new wheel. My current rear wheel looks like this:


So what I would want it would be a 120mm hub that can be spaced out to 126mm, (like the formulas i think?) and then some spacers which just screw on to the axel and space it out the same way that I had before. I'll need to have the wheel trued to the left because of the uneven spacing, right?

Anything else I should know? This website (& sheldon's of course) has been a treasure trove of information.