I believe Windows 7 Enterprise is in effect Windows 7 Pro but for a limited time of use. You can buy a Home basic, Home Premium or Pro & it will accept the key & activate disable whatever features of the windows that are not part of the license but you can't buy Ultimate because it would require more software.
I might be wrong but from my understanding that's what you can do. Or, go on ebay & find a cheap OEM version of windows 7.