For someone like me, with average mechanical skills with a set of tools given to him/her last Christmas, you "buy as you go".
As a teen, I started out with a Sears Craftsman 13-piece metric/american socket set, and was able to do most top-end work with that. Borrowed (and eventually kept) my dad's crescent wrenches, and bought more as I needed.
Then I got some feeler gauges from mechanic friends, got some hex wrenches from raffles at car shows I attended, bought some specialty tools along the way...
...twenty-odd years later, my wife bought me one of those big Sears tool chests with the wheels for Christmas, so now I have all my tools arranged nicely in drawers. When I got the MBs, I found those hex bolts to be quite annoying and would have to stop short of all sorts of projects. So my recent purchase was a a set of hex sockets.
If I decide to tackle the head gasket job (when it happens), I will then get any specialty tools I need to do that job.
I had my garage wired for 220 when it was built, in anticipation of buying a compressor and assorted air tools to boot...then I can REALLY get into trouble!!!
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car