I agree the I/H/E will be a nice setup for the QG18DE, as for the port and polish if you are staying all motor as you have stated then i would recommend that after you have done most if not all "bolt-on's". As for voiding warranty you will, with most dealerships, void the warranty, although there are laws preventing the voiding of a warranty just because aftermarket products are present, the part must be creating the problem for them to refuse. As e2001gxe said
www.b15sentra.net use the search and you will learn, always search. As for me I beat on mine bad when I first got it I mean BAD, I did terrible things to the engine like not changing the oil anywhere near 3000 more like 11,000 miles (I know, I know) anyways she runs great I have learned a ton and have much more respect for my car and if that doesnt scream reliable tell me what does. Hell I am pushing 40,000 and NO ENGINE PROBLEMS. eheh.
PS. 36000 miles comes faster then you think.