The newer OS requires an APFS file system on the main drive of the computer if SSD but does not make it mandatory for an external. What I would suggest is getting an external SSD and installing Mohave through the MBP onto the new external. You can avoid having to make the external APFS. Once installed, boot to the external and use the Mac to move info/data from the MBP to the external as if you were moving to a new computer. After that is done, test everything and if OK, erase the MBP and clone the external to the MBP.
I know it sounds like a lot of work but it is a relatively "easy" way to avoid having to make your MBP have an APFS system. It's not that having APFS is bad but you could have problems with compatibility with other drives, etc. I don't know the extent of your computer hardware and what programs or files you need to access. Keeping the MBP non APFS should keep things compatible though according the some, may not be as efficient as the new file system.
In any case, I would invest in two externals and clone your present system to one before you do anything so you do have a backup should anything go wrong. If all works out, then clone the new setup to the extra drive. My advice is "you never have too many backups."
Suggest you look up articles on APFS vs MacOS extended to get an idea of the possible problems.
