Both FF and Aftermarket CNGs are good. But for FF CNG you have to visit Authorized Dealers only for maintenance and tunings, as mostly the regular workshops don't keep the software for tuning the FF kit.
So, aftermarket CNGs are easier to maintain.
Always go for LXi, as it will be a better option with ABS and Alloys over the standard version RXi mostly available.
A nicely maintained 2007 LXi with FF CNG and with 45K kms on the odo will go in between 700-750K easily.