From your post I can see you need a lot of different functionality. I would strongly suggest you BUY some software. It will work out a lot cheaper in the long run than to have a bespoke system written for you.

If cost is an issue, you can look at free, open source packages that will most likely run on an intranet.

As a previous poster stated, how many employees will use the system? How many transactions do you estimate per month? All these type of questions will have to be answered before you can decide on what software you need.