I want to use Migration Assistant from old iMac (Early 2009) to New Imac (iMac 24") but I'm concerned I may inadvertantly infecting new mac with some unseen, virus or something like searchbaron. What to do?

What you can do is download and run the free version or Malwarebytes on the old Mac. It was developed by a long time contributor to these forums and a highly respected member of the computer security community. 

If the report comes back clean then go ahead and use Migration Assistant.

By the way there is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This documents describe what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community and Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support.  

There are no known viruses, i.e. self propagating, for Macs.  There are, however, adware and malware which require the user to install although unwittingly most of the time thru sneaky links, etc.   

Anti Virus developers try to group all types as viruses into their ad campaigns of fear.  They do a poor job of the detecting and isolating the adware and malware.  Since there are no viruses these apps use up a lot of system resources searching for what is non-existent and adversely affect system and app performance.

There is one app, Malwarebytes, which was developed by a long time contributor to these forums and a highly respected member of the computer security community, that is designed solely to seek out adware and known malware and remove it.  The free version is more than adequate for most users.