No virus protection is needed as Mac OS was designed from it's very beginnings almost 20 years ago to be extremely secure. Most users enjoy years of trouble free operation if they simply keep Mac OS up-to-date. It is unwise to install any antivirus, cleaning or any other third party "maintenance" utilities as they simply are not necessary and tend to create far more problems than they ever solve.
As far as which iMac is right for you I strongly suggest visiting your local Apple Store or Apple reseller so you can describe what your needs are and they can help you match that to the right computer.
As there are two different size iMacs (21.5" & 27" displays) that would be the first decision you need to make. At the very least get one with 16GB of RAM and a Fusion drive. If you want the fastest available then consider instead of a Fusion drive buying one with a SSD, the advantage being speed and reliability as there are no moving parts with a SSD. However again, for exactly which configuration is best for you, please visit your local Apple Store.