I would start with a 6061 aluminum frame (not 7075 as it is extremely difficult to weld) as it would be light and strong. Carbon would be nice too but would add a lot of expense and is more difficult to work with. I would also think cushioning would be a major issue as people spend all day in these chairs so some nice advanced memory foam would be in order. It also seems to me updating to lipo's and a good brushless motor would be in order. I think one or two of the hobby city large outrunners linked below would be good as well.
http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=5139&Product_Name=HXT_80-100-A_180Kv_Brushless_Outrunner_(eq:_70-55)
I would guess that chain drive would be optimal for this as well.
Let us know what you need and I will do what I can to help.