Two things that I would have changed:
1) Plane for the mission. I actually did a great job of getting the plane I needed for my mission at the time. The trouble is, the mission changed when I added family, new job, etc. It won't be too long before I need to upgrade from my C (which I love). Some of this you don't know until you start, but I wish I would have thought a couple of years down the line. (You don't want to overdo this, either, though, or you get stuck paying for a plane that you really don't need to afford.)
2) Better pre-purchase inspection. I was in Afghanistan when mine was performed, and this was the first plane that I had purchased. I didn't know exactly how the pre-purchase worked. I ended up paying for more of it than I should have (seller should pay for airworthy items, you should pay for additions/nice to haves) and from a shop that wasn't too reputable. A year later, I had a separate big bill due at my first annual inspection. (Fortunately, after that, my plane's been in fantastic shape!)