The trueflow inlet pipe will more or less hook up to your stock system so yes you can use the K&N air filter with it. As for the C&L maf meter, it will only work if it is calibrated for your application. Otherwise, your car wont be running on the right fuel pressure and you may have complications.
A larger MAF meter on a n/a Mustang that isnt seriously modified is a waste IMHO. Theres no point in doing it. The gains are very minimal for the amount of money you will be spending.
Lee doesn't carry a TruFlow application for my car, but the C&L MAF comes highly recommended from first-hand experience.
(a) How hard to install?
takes about 15-20 minutes.
(b)Does it make the engine run lean/rich?
in most instances I've heard about, it leans A/F ratios. It also tends to increase HP #'s on a dyno, to the tune of 13 RWHP in our particular case.
(c) Already have a K &N filter with the stock system, can i just reuse it?
the C&L comes with a conical (round) K&N filter. I haven't replaced it, so I can't really tell you exactly which part # it would be from K&N. It is NOT the flat rectangular filter, though.