If I format my USB key, would that completely destroy any viruses or other crap?
Virus' on USB keys can be buried very deep into the root of the drive itself. Read about Stuxnet and even about how some hackers got into the source code of the USB manufacturers system to be able to replace it with spyware/malware etc.
If you think your USB key has a virus, toss it out and buy another one for $20.
But....Just a hunch based on my observations of some of your posts re van's and where you sound like go on the internet when you are not on TERB is that you might be wanting to wipe any images aka "other crap" that you might not want to become evidence.
To that question, the answer is that, unlike magnetic drives, old files written to flash media are readily recoverable even after being wiped/overwritten hundreds of times. It just takes computing power and time. Depending on the value of what you are hiding, LE might not bother.
If you don't want your data to be read by anyone, physically destroy the flash media with scissors and fire.
Then encrypt your entire NEW drive with whatever type/level of encryption you can handle. Highly unlikely it can be cracked without your password and IIRC there is case law against self-incrimination and being ordered to provide your password. So it is likely secure. But if you ever want to wipe the encrypted data, then you should be using a magnetic HDD.
Or just stop hunting and hoarding porn!