Your first machine should be a post bed with a roller presser foot.... you can make a simple platform to get these machines to work as a flatbed as well. Singer models: 51w or even better a Singer 236. But Consew,pfaff, juki and others make good postbeds as well.
Don't take my word for it, read what the experts have to say about what machines are needed to make shoes/boots: Read the post from Friday, December 26,2008 and the answers from the next 2 or 3 days.
http://www.thehcc.org/discus/messages/4/1386.html?12566782253