Outlook for contact management

I am designing journal entries for expense reporting of a sales team for the companies they visit. I have a field for expenses, name of the company and the contact person visited in this company. Is there a way of representing only the names of the contact people associated with a particular company (instead of all the contacts) when the user selects the company from a combo box? Also, is there a limit (by Outlook) on the number of entries I can add in the combo-box? Is there any other method for making an expense reporting for a contact instead of separate journal entries each time? Finally, are there third party softwares that can interface with outlook for contact management purposes?
Thanks
Sanjay