Originally Posted by CyclingDaily

Changing from a purefix to those aluminum track bike is a big jump I.M.O. !
I'd say change the chain first, I never had a purefix but I'm sure they use a KMC chain.
I had a KMC chain on one of my bicycle, it snapped on me while stopping, thank God it wasn't a hill though.
Then I would move on to the crankset along with a bottom bracket, maybe a sugino messenger ?
It treated me well, hill climbing everyday. You will feel a big difference when you really push on those pedals.
Then I would change the wheel set, I know they come with heavy deep profile rims.
Get a new cog and lockring , surly is decent for its price !
Pedals- This is your preference Efficiency < Comfort ? Bmx pedals with straps, or road/track pedals with cages and straps.
Then your stem, handlebars, headset, etc
I made a mistake and started with handlebars first, I kept buying different types of handlebars then I couldn't afford to upgrade my parts.
After all that you can just throw the parts on your purefix on to a new frame !
Hope this helped you decide !