Forget about Windows 8 which is too young to be stable. Even with Windows 7 I get weekly patches. With Microsoft it is always best to wait for Service Pack 1 to be released before going with one of their operating systems.
Windows 7 (and Win8) are resource hogs so it is best to buy a new computer with a more powerful processor and a PC that will accept more RAM (ideally 16GB or more).
With regard to business software all the vendors ported over their code to Windows 7 a couple years ago and it is stable. That includes the problems with IE versions from 8 on that were not compatible with apps like SAP's web interface.
You can order PC's directly from HP or Lenovo with Windows 7 loaded or buy them from places like PCconnection.com that deal with business users.