If that machine's been in storage long, the first thing you should do is CHECK THE BATTERY. Make sure it's not leaking, or you'll have a dead or soon-to-be-dead A2000 on your hands.
Now you can move onto the fun stuff! Either ditch the 1.3 ROMs and get 3.1, or install a ROM switcher so you can use them both. 3.1 will cost you direct compatibility with old games, but you can use WHDLoad to install them to hard disk under 3.1.
You'll probably want an accelerator, too. Blizzard2060 is a good bet, but hard to find. 030 and 040 accelerators are easier to come by. Also note that if you keep your 1.3ROMs in the switcher, you can't use an 060 (possibly and 040 either). Many accelerators have SCSI controllers onboard for a harddrive.
Most accelerators will let you add FAST RAM to them. If you want more CHIP RAM, you'll need a MegAChip, DKB used to make them. Most retailers still have them, but they're expensive.
Next, you'll probably want a GFX card for better screenmodes. CV3D and PicassoIV are probably the best cards for the A2000. Also consider a network card for ethernet access.
Be aware that upgrading an A2000 is extremely expensive. You might be better off part-exchanging it for an A3000. You'll lose the internal 5.25" bay and some card slots, but you have the option of getting PPC acceleration and faster Zorro3 (vs Zorro2) GFX cards. It also has a built in scandoubler so all modern monitors will work natively. If you get an A4000, you can swap the daughterboard with one for PCI slots and get more modern GFX expansions. No scandoubler on the 4000, though. It's your call.
Phew! Have I missed anything?
Welcome to Amiga.org!