I just got a Sony TRV840. Digital 8. The battery which came with the unit lasts 90 minutes. 1/2 of what it can with normal 8. The tapes, you can buy cheapo ones and they look just as good as the digital 8 tapes. You can get two for about 6 dollars. Mini-dv tapes are more expensive.
I'd get the best optical zoom I could. Mine is only 15x but you can buy an optical 2x adapter which = 30x. The digital zooms pretty much suck.
Btw, my 840 makes a loud noise indoors. It is the tape spinning and the microphone picks it up. Outdoors, you can't really hear it. So, if you take lots of indoor movies, make sure you test it out with the tape going.
Also, the quality of my 840 is outstanding.
Good luck.