For business use, you need a stable, enterprise-class distribution. Assuming you don't want to pay for Red Hat or SUSE, there are three:
CentOS (a free version of Red Hat Enterprise Linux)
Debian Stable: for a small computer install with the AntiX disk
Slackware: best experienced as Salix, which adds lots of extra software
If you're still using XP and would need new hardware for Windows 8, then memory size is important. On the desktop, the minimum requirements are
256MB for Salix
128MB (just about) with LXDE version of Salix or the AntiX installer for Debian.
You can get away with 128GB or less on the server if you install without a GUI. That may seem scary if you're used to Windows, but that's the way it's usually done: the less stuff you have running, the easier it is to sort out a problem if one arises.
Have a look here for available software
LibreOffice has the facilities of Microsoft Office and Gnucash is probably good enough for your accounts, although commercial programs are also available.
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