A little more re: battery. You need a diode to disable the charging circuit, otherwise bad things will happen with that non-rechargeable CR2032. A quick google search shows that rechargeable 2032s now exist, so you might be able to use those by installing just the coin socket. I put one of
these in my A2000. It includes the socket and diode pre-installed on a small PCB.
In addition to removing the old battery via soldering iron or snips, you can also just rock the battery back and forth until metal fatigue causes it to give way.
Re: the firmware on the 2091, there should be 2 similar looking chips just next to the RAM chips/sockets and the DMAC chip. If they say 7.0, you're good to go for drives up to 4GB (and higher, with software patches, but best stick to a basic setup for now). Since this was a Commodore machine, hopefully they kept it up to date
The drive on ebay you linked to is overpriced. Just do a search for 50pin SCSI and you should get some cheaper results.
