Regarding SDR 802.11b, keep in mind that functionally, you’ll just be replacing the baseband/MAC IC in the system. The typcial 11b interface is really just a radio tx/rx (like the MAX2820 family), a BB controller (TI or some other up-and-coming vendors), and a TCXO, antenna, passives.
To talk about an SDR implementation, you’re replacing the existing MAC and talking to the radio directly. Talking to the radio, you’ve got:
inputs I+, I-, Q+, Q- (pre-filtered I/Q data — usually root-raised-cosine, alpha=0.7 (not exact))
outputs I+ I- Q+ Q-
SPI interface for channel settings, filter settings, etc
logic lines for tx/rx/idle mode control
analog AGC lines for tx and rx
a few other proprietary controls — maybe an a/g switch for a dual-band radio
Using a DAC to generate the BB signal, something like 8-bit @80MHz should do OK. I’m not sure about the receiver, although people have been talking about 8-10bit at Nyquist (>22MHz).
Vendors that sell 11b radios (like Maxim) often have Eval kits built up that allow control via a PC. In the case of the 282x family, you could write code to bit-bang on a PC parallel port (emulate SPI and some of the logic controls), and then seperately control the ADCs and DACs for the data I/O, as well as the two DACs for the AGC (6-8bit, very slow).