The antivirus doesn't come with Spybot 2.4 automatically.The anti-virus comes with the paid editions of Spybot 2.4:
http://www.safer-networking.org/private/compare/
Or,users that buy the Spybot + antivirus editions can disable Live Protection if they prefer,if they already have an antivirus installed.
No,Spybot 2.4 will not overwrite Spybot 1.6.2.
Spybot 1.6.2 is installed to the Spybot - Search & Destroy folder in Program Files by default,and Spybot 2.4 is installed to the Spybot - Search & Destroy 2 folder in Program Files by default.
Having both installed on the same computer at the same time would be noisy,hectic and (in my own opinion) probably not a good idea.
When you download the Spybot 2.4 setup file,it should ask if you would like to uninstall Spybot 1.6.2,and if you answer yes it will uninstall it for you.
However,I know that you prefer Spybot 1.6.2.
Unless the pop-up stated otherwise,Spybot 1.6.2 is still offered on the download page and appears to still be supported,so if you wish to stay with Spybot 1.6.2,you can.