I'm a fan of Krav Maga as I feel it's an excellent vehicle to take the untrained beginning student and give them real world defensive skills in a very short time.It does have it's limits and is does not cover all aspects(such as ground-fighting) and manual weapon applications with real depth,but it's a good basic combatives system to get started in unarmed defense.
Many schools class time is spent doing "work-outs" & not enough on skill development. New technical skills need to be done when still fresh & ideally under the guidance of a qualified Instructor. Work-outs & stamina training is not complex and can be done outside of class.
Krav defenses frequently incorporate simultaneous defense/attack sequences and the movement is done in a way where both hands are not committed to a single defensive move.Usually fine to do so,but there are many times when two hands are needed to perform a gun or knife disarm, so I would not limit myself to only practicing them one way.
You want Instruction to come a reputable source and I've set up links to all the major Krav Schools.