Few things, if GbV allows taping, get a bigger "non-stealth" rig, its easier. If you ahve to stealth, go with the best mics possible. You don't have the luxury of tons of gadgets in line between the mics and recorder (like preamps, and ADCs, etc.) so get good mics. Personally I feel the mics are the most important part of the rig, being the "ears".
As for ADC (Analog>digital convertor) the AD20 is designed to be portable, though it may not be stealthy (3"x5"x1"). Its probably the smallest, yet excellent quality, ADC out there.
To record AUD cheaply with NJB3:
SPmics>NJB3 (line in) - simple easy.
Add the AD20:
SPmics>AD20>Optical>NJB3 (optical in) - little bit more stuff, just as easy, perhaps better sound, though mic position is MOST crucial. This rig will give worse recordings if in the wrong spot.
To record SBD:
SBD>RCA to 1/8"TRS cable>NJB3. easy simple, just like the AUD.
Good luck!
T