I had a Sundown 3000D on 4 Chaos 15s and the subs never even got the least bit warm, even after full volume and 4-5 songs. Id definately say go for 1krms maybe a little more each.
I would just go sealed. Will still be loud. Something like .75 sealed for each sub would be great.
As for power, those new Audiopipe 3ks cant be touched for wattage per dollar. Another amp that is great for the money that Ive seen is the MB Quart 2000.1, alot of people have been running those lately. The MBs are around $375 new, and the APs are around $325 new. Id run 2 subs per amp either way you go.