What is the best DOS computer I can buy for playing games

You can always just use an emulator for free, all you would need are the game files, which you can find everywhere.

Emulator: w w w . emuparadise . me / emulators / files / user / DOSBox-0.74-416 . rar
Games: w w w . emuparadise . me / Abandonware_Games / 51
Note: To open .rar files, you can use 7zip or WinRar. WinRar is my personal preferene