point-of-sale/inventory-tracking system

Hi. I've undertaken the task of setting up a point-of-sale/inventory-tracking system for the family business, which is a fashion boutique. At the moment, the business is completely non-computerised. We have decided not to integrate accounting into the system yet; but just to stick to inventory-tracking for the moment (our elderly accountant would have too much difficulty with such a fast transition, as would we ourselves).
Alot of the clothing items already have barcodes on them, so I plan on printing up barcode labels merely for the remainder. The shop has only one sales counter, at which I am considering setting up a thin client computer. This would be connected to the PC in the back office (which is running Windows XP) via either an ethernet cable or a wireless connection.
I’m setting up a LAN at home, which simulates the system-to-be in the shop. I’ll use it to do some testing once we’ve decided on what software we’re going to use, and once we have the necessary peripherals (hand-held scanner, cash drawer etc.).
(i) Are there particular barcodes I should be using, or would any standard barcodes be suitable?
(ii) Is there any particular software that you would recommend? Ideally, it would allow easy integration with an accounting system at a later stage. I’ve discovered a number of open-source programs on sourceforge.net, but if you have any suggestions, they’re more than welcome. I have no intention of alloting approx. $1,000 on a Microsoft (or other commercial) product, when all we need can be achieved with a freely-distributed program.
(iii) Would using the barcode labels already on some of the items cause any problems?
(iv) Any other recommendations or words of advice (e.g. recommended scanners, thin clients or other pieces of hardware)?

P.S. I’m well aware we could pay someone else to do this for us, but the quotes I’ve got from local businesses seem outrageous. I don’t anticipate any insurmountable problems, so we plan to try do it ourselves first. However, if anyone knows of any common/likely pitfalls/difficulties, please let me know, or warn me about them. We’re not going to be thick-headed about it; if it’s too difficult, it’s too difficult.

Thanks in advance!