2nd opinion scanner does not have BB, FW.
Recommended are MBAM and EEK. both free and can clean th system. NPE is ok but usually irrelevant when you have used MBAM and EEK. HP, ZAM and HitmanPro are perhaps mainly used to do a "3rd or 4th" scan but usually also irrelevant.
Not to say MBAM and EEK are 100% failsafe proof, but yeah, those two are most recommended.
None provide any level of protection, unless you chose such option by mistake.
in terms of system impact, both function fairly well and not a high CPU and I/O. EDIT: that from a mid range system upwards.
On low end, you will notice quite some system impact in regards to any of the above used specially when scanning the full hard drive and cleaning infections.