Rubbing alcohol helps a lot. Of course, it depends on what kind of stickers you're talking about. If they're printable-style stickers, the kind that are sold under the Avery label, you can soak them in alcohol and that usually deactivates-or maybe 'congeals'-the adhesive.
If you're talking about "Property of..." whatever company that owned it before you, then you're in for some problems. Those stickers are SUPPOSED TO BE hard to remove. They are often basically epoxied onto the machine.
If that's the case, you may want to start looking on eBay for replacement case parts...