Welcome to the MLA! I, unfortunately, don't have a newton to sell, but I can answer your questions.
Yes, you can use an ethernet card with the 2x00s. I have a d-link DE660ct and it works great.
To find a list of drivers, goto http://www.chuma.org/newton/ethernet/ It looks like it's down. So i'm borrowing this list from www.thisoldnewt.com:
Farallon PN895 EtherWave
* 3Com 3c589, 3C589B (according to Harri Hohteri), 3c589C, 3c589D (including -TP), 3CXE589DT, 3CXE589ET
* Proxim RangeLAN2 wireless Ethernet *note* this card is not compatible with AirPort or any other 802.11-based technology
* AmbiCom AMB8010 PowerSaver
* TDK Combo cards: DFL5610, DFL3410, DFL3200, DFL9610
* TDK Ethernet cards: LAK-CD021, LAK-CD021AX, LAK-CD021BX
* D-Link DE-650, DE-660 (can be found at zones.com for US$43.99 plus shipping) [Thanks to Kent Wong] DE-660+
* Fujitsu FMV-J182, FMV-J182A
* RATOC REX-R280
* Contec C-NET(PC)C
* MELCO LPC2-T
* Longshine LCS-8534TB
* Correga EtherII PCC-T
Also there is a guy making a wavelan driver. http://www.ff.iij4u.or.jp/~ngc/eng/newtwave.htm
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Sgt. Thelip
Heavy Weapons Specialist
950 division
Liberated Macs: 12