You can use something like the Lyra2 to connect an external keyboard:
http://amigakit.leamancomputing.com/catalog/product_info.php?cPath=49_89&products_id=207Although this is typically for folks who are putting their 1200 in a tower, I don't see any reason why it couldn't be modded to support an external keyboard with a desktop case.
That being said, those ribbon connectors on the 1200 keyboards are a common point of failure. You might try removing it and reseating it a few times to help it get a good connection. There's also a process for repairing them which I believe involves snipping a couple mm off, and sanding it, and doing something else... don't quote me on the exact process, I can't find a link off-hand, but I'm sure someone who's done it more frequently will be along to explain it soon enough.
That's what I'd do if I were in your shoes - try to repair the ribbon cable for the stock keyboard, rather than kludge together some external mess.

Cheers!
Edit: As far as network cards go, plenty of options, mostly centered around the PCMCIA port. Can you do wired or do you require wireless?