Good morning, [URL] That link explicitly calls out Ubuntu Server 18.04 as a supported OS. There should be no issues with installing Ubuntu to your T640. 
I'm going to add new server to do stress specific work and this server should be T640. I'll insert in configuration a PERC H740P or H730P+ with two disk RAID 1, and would like to be sure that the controllers will be recognized by Ubuntu 18.04 installation, as in previous one purchase the PERC controller was not compatible and i had to install the OS disabling it via BIOS and using software RAID management. I've read documentation in dell site about the compatibility and should be ok, as 18.04 result compatible, but i prefer to be very sure, so i ask if the PERC controllers is compatible with 18.04 Ubuntu installation. I've also found some post that suggest to get UEFI BIOS and not Legacy BIOS as the last one can cause problems in installation. Is that true?
PERC H740P/H730P+ with Ubuntu 18.04