External, internal, amp, headunit, processor....all the same in what they could possibly do. Specific capability is of course related to each specific unit. You reference wanting to buy Zapco. The DC Ref series amplifiers have plenty of DSP to replace nearly any passive (assuming 1 channel per driver of course). The disadvantage of this style crossover is you need a front end/PC or what not to control them. ie, you aren't driving and making changes. While doing what I just wrote at the same time isn't logical for your safety or others, being able to tinker as you think of what you want to tinker on is worth its weight in gold in the learning process. Sure it is completely unnecessary, but that doesn't mean it couldn't be the best money you could have ever spent on car audio. Learning is priceless though. That all being said, I love those amps. Could easily run them. Will do what is needed.
I am a nut though. I NEED control. So much so my cure was to just repalce my headunit with a computer. No need to bring one with and plug it in anymore