AFAIK the Scart Spec only allows up to 576 lines, so you would be taking a digital signal, converting it to a lower resolution analogue signal into your HDD recorder, which then converts it back to digital to record onto the hard drive. When you playback you again convert from digital to analogue to your TV.
If your IPTV box can record to an external HDD via USB and you don't use the old DVD/HDD for anything else, I 'd take the drive out of the HDD and get a USB to IDE or USB to SATA cable (depending on the hard drive). Then you can record digital straight to the HDD and playback without any analogue stage.