Colin, computers powerful enough to do the work a business like ours does need not be an expensive one.
If you have any computer fayres about you could pick up a new box for under £200 which will be a good spec. If your monitor is ok then use it, if you have your operating system cd use it, attach your keyboard mouse and away you go. £200 - and set against business expense.
Its only if you doing high end graphics or playing high end games yo need to spend more money, and most kids have Wiis and Playstations for games anyway.