Hello from a fellow "large saltwater tank to large planted tank" convert. I am just now starting to stock my 300g that was converted from a reef, so I can certainly sympathize with your upcoming conversion.
Regarding flow, you'll probably find 2,500gph to be too much. My tank maxes out at about 1,700gph, but I've got the pumps throttled back to about 1,000gph right now, and with the tank about 15% planted, find that the flow is almost too much right now.
MH are great with plants, but can cause lots of carnage to plants without careful use. Even 2x400W mounted high above the water might be too much, but it's also worth a try considering you have the equipment laying around. As far as colors go, 10,000K bulbs would be fine. I run XM 10,000K bulbs over my tank and have been pleased with the appearance. Plus, I've already noticed a little growth on the plants, and they've only been in there two days.
If you get a chance to browse the Photo Album forum, check out Medicineman's large MH-lit tanks. I learned quite a bit about large planted tanks through his threads. Best of luck with your project!