As long as it's not a Mini Wii, it can be modded. As for "what to buy" it just depends on what you want to do with it. For example, newer Wii's don't have GameCube ports nor do the drives (usually, reportedly some do) accept GameCube discs so you wouldn't have stand alone GameCube support. Granted, Devolution would allow GameCube play, you'd still need an older Wii that could verify the discs.
Secondly, newer Wii's also can't read burned discs. While we recommend USB loading, some people do prefer burning discs.
Thirdly, newer Wii's also cannot use BootMii as Boot2. BootMii as Boot2 gives you maximum brick protection.
So my best advice is to look for a Wii that has GameCube ports. They are most likely to have the best options for Homebrew uses including BootMii as boot2.
If I spelled something wrong, I'll just blame it on auto-correct