"if" is not for looping but branching.
As you said, ftp has a limited command set and does not offer logical branching and other scripting commands.

You could write a shell script that checks the output of a ftp command for example. Since checking if a file is present on another host using ls via ftp for example, will always return an exit code of 0 from your ftp command, so checking with $? is not possible. Though, ftp has it's own status codes which are 3 digits that you could parse, but I think that's rather awkward and not very reliable.

As ftp is not encrypted and sowith not secure, you should also think about the option of implementing the ssh suite, that include ssh, scp and sftp. It does a key based authentication so that you do not have to use clear passwords in a ftp-here script. Also all communication would be encrypted. There are countless guides on the www on how to setup ssh/scp/sftp without the need of passwords. You can also find plenty of threads inside this forum.

Checking if a file is present with ssh or scp from a script is rather easy, once it is set up.