Based on my experience of the last 8 mos, I would discourage any attempt, beyond the most elemental, to use LV PDA with DAQ.
But, if you must, the HP iPAQ 5550 (and a few other models in the series) is compatible with HP's expansion pack accessory. The one we use contains its own battery and 2 PCMCIA slots. I've got a CAN card in one slot and a 6062E DAQ card in the other; together they draw, according to their specs, about 700 mA! Needless to say, the battery lasts about 15 mins before the PocketPC OS pops up a box saying batt is critically low (i.e. 30% charge) and it is being disconnected! At this point, the cards lose power, and, again, according to NI's specs, the analog input impedances may decrease from 1 GOhm to as low as 800 Ohms (if I remember correctly).
This particular PDA has Bluetooth built in, but accommodates WiFi in its SD memory slot. If you go down this road, be prepared for dark, cloudy skies.