1) How much do these run used and what is an acceptable number of hours?
A used D8 will run you about $250 - $300 used. There is no way to reliably track the number of hours on one so people will usually not say. If somebody says that they used it for like 200 hours, I would pick it up. I believe that the heads wear out after 1500 hours, but I am not totally positive on that.
2) If only using it to patch out of a SBD, can I just run an analog line in or will have I have to get an a/d?
You can use an analog line-in without a problem. If you are concerned about the poor A/D quality on the D8, look into the M1 or D100, each of which has a better A/D than the D8. Either of those will run you about $450 used and is an overall better buy because their heads do not wear out as fast either. The D8 uses 1/2 size heads; the M1 / D100 use 3/4 size heads.
3) Are these things complicated?
Absolutely not. Put the tape in, press any button to load it up, press "record" and then "pause" and you are all ready to go!
If you want to patch into other decks with any of the Sony portable DAT decks, you will need a 7-pin coaxial cable, available for about $125 - $150 new from Oade or ProDigital or cheaper used.
As for other options, there really aren't any beside the DAT decks, JB3, or MiniDisc decks. There are laptop options, but I am not sure how much you would run in total with the accessories necessary for SBD recording.