FWIW, they long since stopped shipping with the smaller battery, so you're good to go, there.
eBay is hit and miss- I was lucky, got mine used, with all the upgrades (pre-waterfall), as well as antennas, GPS and external charger with second battery, for $700. Works like a charm!
You will need to consider the DMR and NXDN upgrades, as you'll discover more with these. ProVoice is not as essential, but there's still a few systems using it. The waterfall is a nice, sometimes useful addition, but not required. Do not get these upgrades from anywhere but Uniden.
Definitely, most definitely, you WILL need a replacement antenna, as what comes with the SDS100 is.......just short of deaf.
There are plenty of resources here on RadioRefence for the SDS100, including links to YouTube videos.
This scanner WILL seem quite daunting, as there's so much is such a small package, but after a bit, you'll be scanning like a pro!
And above all else, DON'T PANIC! Search these forums for answers, first, then if you have questions after that, ask! That's what we're here for!