OpenERP 7.0 expert. Please check my profile & portfolios for more information. I used finish such kind of jobs for a super market, and it is not easy as you described.
With my experience, I propose this way:
1. Barcode print: I should by a specialist printer to print barcode. Quite easy, just one second searching on internet.
2. Pos terminal: I write another application on .NET, to connect to openERP. Because the pos of openerp have too much limit: difficult to customize, low performance, not suitable with such kind of high transaction application.
May be my bid is out of your budget,. But just remember me if you cannot find a most suitable application for you, because I used to do it already.
Thanks for reading.