Don't have a booth yet (been planning one for months), but here's my two cents:
1. My recollection is neither Home Depot nor Lowe's has the type of fan I would recommend; the closest I've seen in the hardware stores is probably with the fireplaces (somebody please correct me if I'm wrong). Most posts I've seen recommend something from Grainger, though I've seen people who are happy with muffin fans (the ones that come in computers are too small, but larger versions with more cfm are available).
2. I would recommend metal or wood, though I've also seen patterns for plexiglas and lexan. My experience with plastics (especially the plastic bins) is that they tend to build a static charge, and that may do some funny things with the finely atomized paint.
This post (http://www.finescale.com/fsm/community/forum/topic.asp?TOPIC_ID=17493) has what I think is a good idea, though the cart he's talking about is ~$70 if memory serves. I'm currently looking for an used cabinet or nightstand that I can buy for much less and adapt. Plywood is probably OK as long as you have the tools.
3. Here's a link to a site that talks about fan sizing and spray booth design.
http://www.briansmodelcars.com/tutorials/tutorial.asp?TutorialID=23&CurPage=1
4. You'll probably want a light for the inside... Lowe's has a nice sized flourescent fixture for ~$15, and incandescent fixtures can be had for about the same price.
5. I would also recommend the purchase of a fire extinguisher... just in case.