as far as I'm aware (i haven't done anything differently) is that you can just buy a pc like normal and as long as all the relevant invoice info is there it will be fine with the accountants.
Some software may need specific versions to allow business use but the os and the pc itself should be fine.
Main thing I've seen with 'business' computers is they usually have vat deducted
edit: I did forget one aspect that's different, you don't get the same consumer rights, ie 7 days etc.