If you want a truly custom motor erl, endyne and inline pro are probably the only people worth talking to....personally unless you are a high level racer I would stay clear and use oem parts for the reliability and cost....of course this route will never make you anyone's hero, but your car will be reliable and make gobbs of power...in most honda's anything over 250-275 whp is truly a waste, as the chassis was never designed to hamdle that kind of power
IPS heads r done by Saul Salceda of MissioN Critical Performance. You can see his work by looking at the pictures of the head on my build thread. There r pictures in the first page as well as in detail a few pages later. The head will be ported in accordance to the cams u'll be running. The pistons and rods r a diff equation entirely.
If you want a K20 head I would get the K20Z3 (06+ Civic Si) head due to the coolant ports. Here is a thread describing the differences: http://www.k20a.org/forum/showthread.php?t=41971&highlight=coolant+divertor
Now, if you are going to have the head CNC'd to match the pistons then you can choose either the K24A2 or the K20Z3. The smaller ports on the K24 head can be opened up while the combustion chambers are cut.
Well, I would first pick up an 06 TSX motor. The TSX crank is the strongest of the OEM K24 cranks, and the head has some nice features (+1mm oversized valves on the intake, more aggressive cams) that you can resell pretty easily if you don't want to use them.
Have you seen this thread? http://k20a.org/forum/showthread.php?t=71489
Good simple build. You can do something like that to start, and have a second head and custom pistons made on the side for a future install.
You can pick up the 5-speed for cheap, then install the TSX 6th gear and make it a 6-speed. The trans cases are the same. 4.7 is what i would get. If you need OEM parts hit up member rbsjnh. He is super cool, and can work a good deal for you.
You can tear through that 15k budget fast if you buy a lot of things you don't really need.
What parts do you currently have?
I would most likely run a 5 speed if possible...less shifting, unless you'd advise against that. I do want to try and get LSD. My budget....sky is the limit, but I will spend with caution when exceeding 15k
Generally, every percent increase in displacement will increase torque the same percentage across the power band.
Going bigger requires a larger piston, so the extra weight will decrease efficiency at higher RPM.
Going with a custom piston will allow you to get larger diameter and lighter weight. If you really want a good piston you will have the head and piston designed together so that they complement each other in the combustion chamber.
I would start with a 87.5 x 99 K24 build. It will be just enough material removed from the cylinders to ensure they are true round while maintaining the bore separation/head surface area required for forced induction or nitrous applications that may come later down the road.