Giving it a shot with trial accounts is an excellent idea and the best way to get some feedback for your actual machine.

I can't say what's the lowest spec machine someone has boxed 5 accounts on - but I can this it depends on what your requirements/goals are. If you can accept having very low resolution & refresh settings on your 4 slaves and just living with one main screen, the requirements will be much lower. If you plan to be able to switch leader among toons, which frankly is pretty key, running 5 accounts on one machine is pretty darn demanding on the hardware. For myself, using a quad-core CPU, 8 gb RAM, and a solid-state drive for the data cache, I considerable my system performance basically adequate. Not great. I still get substantial lag in Dalaran, for instance.


As far as starting zone versus instances versus major cities for testing, instances aren't really any more demanding than the starting zone, and may even be less demanding. Major cities is another matter altogether. You can bring the trial accounts to Stormwind/Org though to test this out; while not the lag fest that Dalaran is, these areas are still fairly busy at least at peak times on my server.