Yeah, that's why I asked if it was going to be a top feed system. Heck yeah, it'll circulate just fine. I don't think whether your return lines are individual, or manifold style matters as all. I think they just put them that way to make em look nice, and neat. As far as functionality goes though I would rather have individual lines so you can move the buckets around if you need to. As long as you are top feeding, it has no choice but to circulate back to the main bucket. Very simple design, and works GREAT. Recirculating Deep Water Culture is a bad ass system. Are you still planning on having it flood, and drain? That's not really necessary. I think the Deep Water Culture will out perform an ebb n flow any day. I would put an airstone in each bucket.
You could have saved quite a bit of money by just using rubber grommets, and barbed fittings from Lowes or Home Depot. The grommets are only like 30 cents, and the fittings are not much more. They push right into the grommets, and I've never had one leak. You don't really need a huge pump for a top feed system, but what you've got is probably perfect. I used a smaller 250 and moved enough water to make the levels in my bucket uneven because I didn't have individual drain lines which is why I say they are a good idea.
Having the pump internal, or external doesn't matter. It's a great system, but if you decide to add on use rubber grommets and barbed fittings from the hardware store. Sucks you put so much money into it, but it'll be a good system and you learned a lot. It shouldn't cost you more than 10 bucks including everything per bucket to add on if you use hardware store stuff.
The first system I build was an under current. I vowed that I would go with top feed after that. It worked ok, but the middle bucket was always a bit higher than the two side buckets since the pump was pumping into it. Bigger drain lines would have fixed it, but I retired it after a while. Next system will be top feed recirculating.