You don’t have to go that far (although by using the El, you certainly could.) There are cheaper places in downtown...Club Quarters, La Quinta, Hyatt Place, etc. Probably a mile or less from the Double Tree. I would just do a search on a travel website and look for the cheapest rate in downtown if money is an issue. (Orbitz, Kayak, Hotwire, etc.) There’s also a hostel downtown if you don’t mind sharing your room.
I'd say it's more casual than business casual. I usually wear jeans and a polo shirt. If you want to dress up you can wear Dockers but it's really up to you. I try and go for comfort but I think a polo shirt would be better than a t-shirt.