Here is a list of recommended hardware to run Plan 9. All these machines are known to be supported by the current Plan 9 from Bell Labs, NIX and 9k distributions but could require some minor patches. They are good-quality hardware that can still be bought today.
If you are installing Plan 9 from CD-ROM, you may want to add the sdiahci driver to the pcflop kernel, since they are missing from the default configuration file. A patch is available.