Clamxav is one of the best free anti virus for Mac although the newer systems have their own security software built in.
Be sure to regularly use Software Update to install the latest system security updates to the built in security system. This can be set to check automatically and download any updates.
iOS, as used on the iPad, iPhone and iPod is inherently safe when used with Apps downloaded from Apple's App store. Problems occur when people have had their phones "Jailbroken" which can negate the security system, or have downloaded Apps online and not from the App store.
Having said that, in more than 30 years of fairly intensive professional Mac use I've never had a virus and have only come across one on a client's machine which was contained in a MS Word document that had been loaded on a USB memory stick from a PC.
The best place to buy Macs is from the Apple store but look in the "Refurbished" section. These machines are actually better than new as they've had a full diagnostic check carried out, any out of spec parts replaced and come with the same length of warranty as a new machine. Up to 20% savings in some cases.
Bob.