Good find -- this is a nice deck. I used to run one awhile back, along with an Aiwa HDX3000 (aka, the brick, same as the Marantz PMD700). As you said, full size heads and a rock solid transport.
The digi-I/O is a 3 conductor TRS on 1/8" mini. A radio shack stereo 1/8" mini to RCA adapter can be used to convert this to more typical coax digi-in/out. I forget which "channel", right or left, is digi-out and which is digi-in, but it shouldn't be too hard to figure out. (R-S part # is 274-883.)
I'm sure the original SLA batteries are long since dead, I think this deck is over a dozen years old. The 7.2v li-ion should work fine. I used a 6v SLA battery to run mine in the field. With the 6v, the battery low indicator (flashing) would come on fairly early, but a 6v 7Ahr battery would last about 5+ hours. I think the deck draws about 1000-1100mA for playback/record. So the wallyword li-ion should last about 5 hours as well.