Thank you for your post.
FileMaker Go can open and use FileMaker Pro files. The key here is that the file will have to be created with FileMaker Pro and then either host the file that can be accessed by FileMaker Go, or download the file to the iPad so FileMaker Go can open the file locally.
FileMaker Pro would allow you to create a field where you enter the first five digits, and then another listing would appear with that product grouping where you could then select the specific product. This would require some scripting.
FileMaker Pro does allow you to search all fields. You are not limited to just one field.
Bento is not a relational database product, although it does have some relational capabilities. Whereas FileMaker Pro links tables together via key field(s), the linking of data is all manual in Bento. In your case, this may work. Bento has a Related Data field type that allows you to link records from another table (called Libraries) to the current table, where you bring up a list of records from the related table, enter the 5 digits into the search box, and this filters the list down to those records that meet the search criteria. You then select the record you want to link and click "Save Changes". The record in the related table is then linked to the record in the current table; not key field needed. This information will sync over to Bento for iPad or Bento for iPhone.
I recommend downloading a 30-day trial version of Bento from www.bentotrial.com, and see if this works for you. If you run into limitations, feel free to post on the Bento forum at:
Let me know if you need clarification for anything I covered.