Be careful to match the model number of the PSU. Many of the mass manufacturers used custom sized PSUs such that a standard ATX PSU will not fit. Additionally (if you thought you could buy a new case and PSU) the mother boards and other bits are not standard either.
You may be relegated to buying an exact replacement for the PSU as a dell compatible replacement.
Otherwise you could consider building a computer and scavange the bits that you can - or you can buy a new PC.