"Must have" is pretty subjective. Obviously with that machine, video, and then ethernet, would be your highest priorities. But it depends what you want to do with it.
If you want to use Photoshop, then obviously you'll want to go for one of the higher-end DSP accelerated video cards, and fast SCSI. If you don't see yourself networking this machine much, you could use Appletalk when needed and free up another Nubus slot for something else. If you want to do any music work, then a better soundcard and fast SCSI would be a must. On the other hand if you want to do any analog to digital data capture, you could look for an old Nubus DAQ (Data AcQuisition card) from National Instruments or Hewlett Packard etc.
As far as soundcards go, the Digidesign Audiomedia I and II are pretty great, and cheap. The MediaVision ProAudio Spectrum 16 (Nubus or LC PDS card with breakout box) is really nifty, in that it gives you four stereo inputs (mixed by software-controlled hardware in the BoB down to one stereo A/D converter), MIDI in and out, and a headphone out, as well as a PC joystick port :?: for some reason. However the analogue converters on the Digis are much nicer, and the AM II includes digital audio I/O (RCA S/PDIF)
There's also the Digidesign SampleCell Nubus, which is a hardware sampler with 8 outs and a nice Mac GUI.
Course you could always save up several hundred dollars and try and score one of the Lexicon Nuverbs than come up for sale every, oh, two or three years [

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http://www.shobaffum.com/iici/upgrades.html
http://www.lowendmac.com/video