it is impossible to truly securely delete data you already saved prior to FDE - though I doubt with a typical suicide that anyone would have reason to pay professionals to do data recovery, so that step may not be necessary.
True. There are many good, free utilities that can wipe data more or less securely off a hard drive. You can do a
web search for them. If you actually have data that someone is willing to pay a company to do data recovery to get back, physical destruction of the hard drive or SSD is the way to go.
On the
encrypted data storage side, If you've got data you've just got to have around until the very last minute, you could use a utility named
VeraCrypt to create encrypted files within your existing file system and save the data there, or use it to create encrypted partitions and save the data there.
After you've wiped everything, you could also use a
live operating system that runs directly from a DVD drive or USB key to access the Internet. The idea would be that once you shut down the live operating system, all the data (which is usually stored in memory, rather than written to disk, when you're using a live DVD) goes away. One such live operating system is called
Tails, and it runs all your internet access through anonymizing software.