Or, think of using encoders and a proper VMS server, instead of a DVR. We did this for a project where we took 64 analog cams off of DVRs and added them to the existing 20+ IP cams to a new DW Spectrum server (two actually). We have since added more IP cams, and are installing a third server this month for more additional IP cams this year.
The bonus for us, while using DW Spectrum, was that the licenses we bought for the analog cams will be valid for their IP replacements when that day comes.
We used Axis M7016 encoders, but you may look into other alternatives if cost is an issue.
Also, using DW Spectrum, we can now view all of the cameras from a single client login.