Would this stop almost all buffer overflow and related hacker attacks on a network HELP?

No, This is referred to as Security by obscurity in the computer industry and is highly frowned upon

An open source FTP client would be referred to as 'security by visibility'

Although there may well be exploits published on open source client, you have the advantage of many hundreds, thousands of users continually testing and reviewing the code, and you have both black hat and white hat hackers trying to exploit the program. There are numerous security updates that fix any attacks against the program. So it's almost proven to be stable over the course of time, of course there will always be more attacks but assuming you use the latest version and common sense you will be protected

If you use your own custom written FTP server, you will not have those above advantages, there's no way you can compete.. ..so you will have many more holes that are going to be ripe to be attacked, even though you can't see the source code you can still get an idea of what to do to comprise it.,. It will be much EASIER to execute buffer overflows on simple programs such as yours

A skilled hacker knows enough information about FTP protocol and gaining entry to try some obvious things that are common with FTP in general first and your program won't be prepared for this as it hasn't been hardened

For other programs the only advantage is the attacker hasn't seen this before, but for FTP it won't help you,, it's generally always considered a bad idea