With a MAF adapter from Autozone, an SE-R intake will fit.
Any exhaust designated as for the Sentra non-SE-R will fit, though SE-R units would fit with adjustments to the hangers.
BRM, Greddy, HKS, VRS, and Stromung all make non-SE-R exhausts. Check out our sponsors and you should be able to find at least a few options.
http://www.nicoclub.com/subpages/addirectory.shtml
BRM is owned by the site's owner. You can call them via the number on their website.
And as for the sensor, that seems odd, but if you're not in the VIN range I guess you just weren't lucky that time.
Welcome to NICO! Do some searching, there have been quite a few lengthy discussions on QG18 performance. You may find more mods you're interested in.