I suggest you use paper for now. Have an inventory log for each lot received, and instruct people to log items out and in. Periodically review the actual inventory and log sheets to determine when to reorder. I have tried to manage inventory in Excel, and although it can be done, it seems like it always ends up a mess when working at a startup. Once you have systems in place and they are robust, then you can introduce an electronic inventory system and feel confident it will work.