This cat will definitely be an indoor cat.  I found a couple of listings online regarding plants, and there are several plants (azalea, lily, daffodils, rhododendron, tomatoes) that we have outside that are poisonous to cats - so kitty stays inside.  Fortunately my African violets are on the non-toxic plant list 

  Plus, I wouldn't want him running off and getting lost or being hit by a car 
Speaking of which, how do we keep kitty from running out the front door?
I'm sure I'll come up with a good home for the litterbox and food/water.  We have a big eat-in kitchen (with pergo floor - good for mopping up messes) and we could possibly put the food at one end and the litterbox at the other... hmm.
I like the HEPA filter idea.  We already have one, which we will run while trying out the cat; we'll probably get a second one if we keep him, and I'll look into the Allerpet.  (I already know it will be MY job to bathe the cat! 

 )
As for kitty going upstairs, it would be more accurate to say that he won't be allowed into any of the upstairs rooms (which all have doors that we can keep shut).  I didn't think we'd be able to prevent him from running upstairs, but when he gets to the landing, he will quickly get bored and hopefully come down again 
I am also an avid knitter and cross-stitcher... I'm thinking that all those supplies should be squirreled away upstairs where he can't get at them and swallow a needle or get tangled in yarn...
(oy... it is just like baby-proofing a house!)