I can't imagine that running the BitPay app is very hard, just about any tablet should work. The question is: how much quality do they want as a business and what are they going to do with it.
If it's just going to sit on the counter all the time, plugged in and never get used for anything else this should do just fine:
http://www.biglots.com/p/d/electronics/proscan-android-internet-tabletI think the bigger challenge will be getting it placed on the counter in a way that is convenient for customers and employees. Does it need to swivel? What angle is best? Who's gonna install and or build the "holder?"
Yes, you're probably right. Most tablets would probably work and I think to introduce somebody to BTC they might not want to spend too much.
When I went to local restaurant that accepts BTC, they just brought the tablet out to the table, so it would depend on the type of business. A holder may not be necessary, but you might have a point which I'll have to consider.