These days, there are a lot of universal options that comes close or surpasses the performance of customs. So it's hard to say for sure if customs are worth it. For example, my UE 900 is better than my Sensaphonics 2X, and my 2X is better than my SE535. Price point wise UE 900 is only $399 and Sensaphonics 2X is $750 now. I would even go as far to say that my UE 900 surpasses the performance of the old UE-10 Pro.
Personally I think the decision between custom and universal is more and more about ergonomics and not just about sound quality. I can wear my customs for a much longer period of time without irritation as compared to universals. For some people with certain ear shape, their desired universal IEM will never work. For example, UE 900's casing can still be too big for some people to fit into their ear. I think this is closer to your use case more than just "what sounds best."
The amount of sound isolation is dependent on the material of the custom IEM. Hard acrylic shell and soft silicone shell not only blocks different amount of sound, but blocks differently across different frequency. In general soft silicone would block more sound than hard acrylic. Also in general either would block more than universal fits. However the perception might be different, for example universal might block more sound of people talking, while custom might block more low rumbling engine noise (totally just an example, I've done no subjective test of this whatsoever).
What is reasonable to be driving from an unamped iPod? I've used IEM north of $1000, and there are qualitative difference that's easily identifiable to IEM that is lower in quality. Keep in mind IEMs are designed to be driven from very low powered sources (musicians, on stage, wireless transceivers), so it doesn't take a whole of power to drive the "difference in quality" out of them. Of course they will sound better if you have a good amp, but this isn't the same situation as full-sized headphones that sound like crap without an amp and sound like the gift of god when amped. You *will* hear a difference even at low power.