Welcome!
You've made a good choice with the 1200 - getting it up to spec is much easier than with a 500.
Your top priority should be getting a hard drive. Almost any 2.5" IDE drive for a laptop will work, but you'll have an easier time if it's less than 4GB in size.
Open up the case and check what's inside. If you're lucky, there'll be a metal drive tray to hold the hard drive. You can do without one, but it's safer to have it. You'll need a 44-pin IDE cable to make the data connection to the drive.
Verify the kickstart version too (2 horizontal chips near the big expansion connector). Version 39 corresponds to Release 3.0, version 40 corresponds to Release 3.1, which has bug fixes and makes further expansion easier. You'll also need the 6 OS disks to install the system to the hard drive.
Your next priority should be an accelerator. An 030 board will do, but if you're dead set on the system, you may want to wait until an 060 board turns up.
Aminet has loads of free software. Check here first for anything.
WHDLoad will allow you to install many games to hard disk.
Depending on where you're located, a variety of dealers sell most of the things you'll need.