Hello,
this is a real bid.
I went through your requirements and can come up with a very simple proposal following:
Respect to responsive web, I recommend to use Twitter Bootstrap framework. It's simple, elegant, quickly to build, and support responsive really well.
In case you want to go with relation database, Postgres is a good choice. I will tell the reason later, but you will need some tables:
- Users (store information of staff and vendors)
- Category (for both staff and vendors selection)
- Items (included ItemCode)
- Expenses (this hold a lot information, include above)
There would be additional tables for storing temporary data if necessary.
I suggest you should go with Ruby on Rails. Because it's useful to develop a test module, easy to scale when you want to add more functions. Moreover, you can find free hosting (heroku) with complete application server + database server for your test app. This is the reason I also recommended Postgres.
About functions, there will be dedicated area for vendors, staffs and admin. Admin will need to sign in with predefined username/password (encrypted, certainly), and can manage all vendors, staffs, inputed expense (approved, monitor, remove ...)
I think the complete app can be done in 10 days. During the development, I will deploy code everyday so that you can give feedbacks quickly, to make sure you get the app function as you expected.
Please let me know if I miss any important thing.
Thank you,