RE: What IBEC to buy?
You can use a linear regulator if you can live with the inefficiency of converting battery power to heat, supply enough peak current during CDI charging pulses (measured with a current probe on a scope - nota DVM), and can support 2.5+ amps continous as that is what some ignitions draw. Pretty much everything you said is correct except for the opto isolator. The noise is going to radiate through power and ground and from the wires... Not through the CPU and the signal wire... How does that common mode choke work on differential mode noise?
Linear are much cheaper to build.... It would probably be a good idea for me to offer a linear version at a $25 or $29.99 msrp for the people who don't know or care about the difference. There is no noise from a well designed switcher...only poor designs have noise.