A okay quality DVB-S receiver can be had for far less cost than a PC electronics downconverter. All that likely would be necessary is a preamp at the antenna. I've seen PC card DVB receivers go for $10 and used DVB-S receivers go for $30. New ones, non high def, are less than $100 at the low end.
The disadvantage of ATSC is the complexity and the fact that you are transmitting on a 6 MHz channel. A DVB-S transmission can be done in good quality at about 2 MHZ. Also QPSK isn't going to be nearly as touchy as 8VSB as far as linearity is concerned. I've experimented with both-- I have an e-bay ATSC modulator/transmitter (formally used for demos at "Best Buy") and I find the fiddling that has to be done with ATSC (such as PSIP) data to be a big waste of effort.
The only disadvantage with DVB-S is multipath as QPSK/Satellite isn't really designed for that.. but if you are going to use gain antennas on both ends who cares? The MPEG-2 DVB-S or S2 standards are also far more lenient of audio and video rates. I suppose if you'd like to be cutting edge DVB-S2 and MPEG-4 H.264 would even be better than ATSC.... It's possible to do HD at low rates with H.264 and still only use 2-3 MHz of bandwidth.
I do hope if you are going to use ATSC for a repeater output that you strongly consider multiplexing multiple input receivers.. otherwise you are literally wasting the spectrum that the null packets consume.
Everything Fred says is true on its face. I still, however, believe that ATSC for amateur TV is not a completely wasted effort.
Although DVB-S receivers can be had for cheap, DVB-T would be the better mode for terrestrial reception (it is, after all, what European broadcasters themselves use). DVB-T receivers not only would be uncommon here in North America, they'd likely not be set up for the U.S. Amateur bands. DVB-S receivers also are going to require some fiddling to get them to work.
But some ATSC receivers are frequency agile enough to be able to work at least on the 73 cm ham band (I'm thinking mostly of computer controlled receivers, like the HD HomeRun or DTV receiver cards). I have some hopes that the HD HomeRun may work on the 900 MHz band as well. We'll have to see. But even if we have to use downconverters to tune alternate bands into the receive range of a traditional ATSC receiver, those downconverters could also be used for analog TV as well. And, as Alton Brown is fond of saying, I look down on unitaskers. :)
We hams have been extraordinarily lucky that for the past 30 years or so cable ready TVs were able to receive 70 cm ATV transmissions completely without modification. I wonder how many TV hams we'd have today if it weren't for the fact that anyone could take a consumer TV, attach a different kind of antenna to the input and tune in the local repeater. With the transition to digital, the chickens are coming home to roost and this happy coincidence is going to go away. Because most (but not all) ATSC receivers are not frequency agile enough to tune amateur bands, we won't be able to tell interested folks that all they have to do is tune their TV to a certain channel. But the opposite side of that coin is that in a few years' time after the transition is complete and the last generation of analog TVs is put out to pasture, there will be a boat load of old ATSC converter boxes that potentially could be modified (perhaps simply by supplying alternative firmware) into amateur TV decoders. There's also all of those PC controlled receivers that are frequency agile.
Fred's last comment about bandwidth is also true - and it is certainly the case that the intent is for the system to multiplex multiple inputs eventually. But you have to walk before you can run. Eventually, I'd like to see the system have 4 video channels: an outdoor camera, an analog TV input, a digital TV input, and a system information slide show.