Buy Android phone, install Android from source, have Nexus S -like experience?
In short, you can't just install a stock image from the Google source on any phone because to produce an image, you need all the drivers for the individual phone's hardware. If you can get hold of these, then it is possible to build a stock image but a lot of manufacturers don't release them.

For simpler solution (if you don't want a Nexus S or Nexus One) get a phone that can run Cyanogen and flash the Cyanogen image to the phone. This is pretty close to the pure Google experience, in that it doesn't have carrier or manufacturer modifications on it, but does have some Cyanogen extras included.