Adding products and sorting out discount codes ‘should’ be fairly straightforward. Hikashop, if that’s what you’re using, looks like it supports discounts and adding products should be easy enough. Checking everything’s setup right is just a case of working through it all and testing everything thoroughly. If you don’t mind me asking, who put the site together originally as this is the kind of thing where you would be best off going back to them for.
Although saying that, if the above is what you want doing I’ll happily quote you for it. I’ve setup a fair few Joomla sites and am in the middle of setting up one using nBill and the Barclays payment gateway so I’ve certainly got some relevant experience in this. I’d probably quote on an hourly rate rather than a fixed price though. Let me know some more details if you’d like me to give you an idea of cost.