Good for you, it’s funny the number of people thinking they need high-end machines for tasks a basic M1 would piss on from a great height. Smart move IMHO.

Just buy one. Whatever you want to spend. Then stop worrying about whether you chose well and start using it for whatever purpose you bought it for. 16GB should be your baseline on anything with Pro in the title but beyond that it’s up to you how much or little you want to spend.

If it’s a 2013 it already has an SSD.

No “virus” or malware is going to survive a reformat on a Mac, simple as that. It can’t “lurk” anywhere one partition tables have been destroyed.

You’re SOL if it’s a 4GB model as RAM isn’t upgradeable but it should still be useable under Catalina if Big Sur performance isn’t good enough. If it has 8GB then you have a better chance.

As you’re asking a question like this I’m guessing you’re not au fait with Mac troubleshooting. Start by going to /Library and look for items in /LaunchAgents and /LaunchDaemons. Use Finder’s “Compress” option to zip up whatever you find and delete the original items. This will stop them loading at startup while allowing you to restore them if needed. In System Preferences go to Users & groups and look at your startup items. Delete anything you don’t want. Finally give the system a Safe Boot as it will perform a drive check and clear out various temp files in the system folder. When you’ve done that you can give Onyx a run at clearing out various caches. It’s the only utility of this sort I trust and use as it’s been around for years and doesn’t introduce any background services or unwanted extras unlike some heavily advertised Mac utilities.