why on earth would he want to get his own software developed? That would be just daft. Im in the epos industry and the systems now need to be have a lot of functionality - some of these packages have been developed for years and we are still requesting new features on a daily basis. There really is no point re-inventing the wheel.
When sourcing your software, make sure it can look after eat in and takeaway vat shifts(assuming you have an eat in area) - take special note if using systems that werent devleoped for the uk.
I would have suggested ICRtouch software setup by a local dealer, but this would be out of your price range. To be honest for epos software i would be looking at £400 - 500 per pos licence with £200-300 setup and programming (which would include valuable support). We sometimes get customers coming to us with cheap touchscreens they bought online, but when they factor in our additional costs to get software, get it setup and running it would have been cheaper to buy from us in the first place!
Obviously im biased though
edit - just a point with regards unix/linux etc - make sure you can get a touchscreen driver that works with your screen before spending too much time loooking at this. There are a few different screens used and they are not all the same and generally need different drivers to work and allow you to calibrate them. If you dont have the right one, it wont work.