That won't give you any HDTV.
To carry any HDTV, you'll need to decode everything on a channel down to a digital transport stream, then re-modulate it as 8VSB or as QAM. Both are expensive...the last time I looked, it was about $1-2K for a QAM modulator, and about $8K for an 8VSB modulator. You'd still need to demodulate the signal, too.
Here's a site that has "white papers" on some of this stuff:http://www.rldrake.com/
Also, you'll need to know whether you want to run "OTA mode", which requires ATSC/8VSB, or "Cable TV" mode, which is QAM. Sets won't usually tune them interchangeably.
"OTA" would allow you to easily mix in the local stations. But, you'll only have room for 68 channels...and, maybe only 50 if the manufacturers cut their tuner programming back to the "core channels" (2-51) later on.
"CATV" mode will allow, maybe, 125 total channels, but then everything has to be converted to QAM.
You can mix analog NTSC channels in with the digitals, as long as you set them up on the right channelization, and use the right modulators.
So, you will probably be looking at $2K per channel, minimum, to do a fully-digital system.