there has been, currently is, and always will be malware against their platform.
I totally agree, but as we already said the probability of infection is several scales of magnitude smaller, because:
A you probably know what you're doing well enough to make your system pretty hack-proof.
B most viruses target Windows, because it's on something like 90% of consumer PCs
I'm not saying that you're just as likely to get infected using linux or an R-Pi as you are with a windows PC. That isn't true. However reduced risk != no risk.
I've only ever had one machine completely taken over by spammers. The machine was a linux box with minimal software installed and locked down
Yes, but that is different to what the OP asked. Yes there are plenty of people who will try and hack Linux boxes (and some get on), there are security exploits (which is why you should auto-update the security fixes). There have been a few instances of trojans too (good practice will help prevent them).
I don't think installing ClamAV will help all that much. Firstly it is mainly used as an email gateway scanner, and secondly just how many Linux ARM virus signatures does it scan for?
As far as I am aware there are very few real Linux viruses out in the wild. See https://help.ubuntu.com/community/Linuxvirus
for details on ones that have documented. Furthermore to get infected requires either a security exploit / hack attack or a bone-headed move by the user. Privilege escalation in order to infect system files is incredibly difficult.
Also remember these are viruses for x86 Linux and so will not run on ARM at all.
Main things to remember
1. install security updates regularly
2. do not do anything as superuser unless it is needed and you know what you are doing.
3. the Unix culture is not to swap executables. It is to pass source around. Of course you can still get infected that way - but you have the source code. Read it if you are unsure of what you are getting. If you can't read it, don't run it.