I would use either a fast '020 board (like Jens') or an '030, personally, and at least 4MB of fast ram. 3.1 ROM's fix a lot of bugs in the 3.0 ones and make working with ACA accelerators specifically a lot easier. And c'mon, they can be bought new for under $20 bucks.
Indivision will allow you to use almost any modern monitor, TV, or projector with your system. Of course some people don't care about these things.
WHDLoad makes running almost any old game like... Well, like it should have been in the first place. Double-click on the icon to start the game, then press the 'quit' key to return to Workbench when you're done. Personally I would never use anything else.

Also ditch the hard drive and get a CF. Kipper2k makes a 90-degree adapter that allows you to access the CF from outside the case.
