In terms of cost vs features the ready built DVR's really can't be
beaten IMHO. The have dedicated hardware and mostly run Linux too but
a very stripped down and lean version. What these need is a fast HD
and no unnecessary services tying up the CPU - quite often hardware is
custom designed for the job with the OS in ROM for 'instant on'.

If you are thinking more of the challenge then go for it, but don't
expect it to be a simple job and get to grips with programming rather
than installing pre-built software