What addin cards should I look for for an Apple IIGS?

You will want to get some more menory; the 1MB card that fits in the auxiliary memory slot is very common. Other cards like 4MB expanders are sometimes available, but expect to pay a premium when they do pop up on auction sites. Which is also the answer to your question about "where to buy" in general...

You will probably want to get on the list for a CFFA3000 card.

And of course a new battery will be helpful - you might want to do a little surgery and replace that expensive battery with a double AA holder from your local electronics retailer.

You'll want to take a look at ADTPro and associated serial cabling for transfering disk images you might find on the wider Internet - which is probably simpler than doing the imaging on the 6100, but that's really down to personal preference.

A large and dedicated Apple II following is available on the comp.sys.apple2 newsgroup.

It certainly is possible. You'll have to look online for suppliers of the chips. You'll need to add jumpers to "tell" the card how much memory is installed. You might find this manual helpful.

Here is the greatest update for the early apple computers:

http://dreher.net/?s=projects/CFforAppleII&c=projects/CFforAppleII/main.php

It allows you to use a compact flash drive as mass storage for your antique computer. Cool stuff.