You should not have to convert the excel file to FileMaker, you can import directly from and excel file into a table in your file.
Instead of perfoming a find one record at a time, you can define a relationship between your two tables:
contacts::FirstName = ImportTable::FirstName AND
contacts::LastName = ImportTable::LastName
You can then add as many fields as you need to see and work with from Contacts to your ImportTable layout. If there is a match between the record in ImportTable and a record in ImportTable, these fields will show the contact data from the matching contact record. If you enter Find Mode and enter an * in the contacts::LastName field on this layout, you'll be able to pull up a found set of all records where the first and last names match so that you can edit and evaluate them.
Once you have finished that part of the process, you can use Import Records to Import all or just some of the Record in you ImportTable into contacts. THere are a lot of options to choose from when importing records, so read up on the process in FileMaker help and make frequent back up copies so that you can start over if you don't get the results you expected.
Note, there are also more sophisticated relationships and/or a filtered portal that can be set up to search for matching contacts where the names aren't quite identical such as two forms of the same first name (Phil, Phillip, Philip, for example).