might want to do a coolant flush and change the intake manifold gaskets. just run the stock plugs, I've had good luck with autolites, other people like the ngk's. with that many miles, I wouldn't plan on modding it.

instead of the downpipe, you could consider sd headers because they cost about the same as the downpipe with the converter, but most sd header kits dont come with a converter, so you might have to reuse your stock one or purchase an aftermarket high flow converter.