Ok, you're actually really well setup to run an active system. It sounds like you have experience or are willing to fabricate, which will help. That Eclipse will allow you to run a 3-way (2-way frontstage + sub) active crossover, which is the way to go if you DIY a frontstage. I would look into something fairly basic, as it may not be worth throwing in ultra high-end stuff for your first attempt at active, and with DIY drivers your money can go a LONG way. I would try to toss in 7" mids if you can make pods for them - I would look into Peerless Exclusive for moderately priced, excellent mids (~$150 a pair from Madisound). Dayton Reference mids would also work well for a budget choice (~$70 for a pair from Parts Express). I would run 2 channels of the amp to these mids. Then, I would look into tweeters. Dayton Reference RS28As are a bit big, but are great for ~$90 a pair from Parts Express. These tweeters will sound great and play low enough to match up with mids in a 2-way setup. I would find a small (25-50W @ 4 ohms, maybe 75W - shouldn't be expensive at all for a decent amp) amp to power the tweeters - you don't need much power for tweeters, as they can deafen you with a watt or two. Then, you will use your HU to cross the speakers. This will be cheaper, and definitely sound better than running 4s as your only mids.
Edit - Alpine lists that amp as 100 x 4, not 125 x 2. I would bridge the rear channels to 300W @ 4 and run that to a dual 2 ohm IDQ or similar sub with great SQ for ~$150.