Super jealous of your sti man. Here's a grit guard that I found online after searching for like two seconds lol. http://www.autobarn.net/gritguard.ht...FUlxQgodAAcAvg
mudflaps-buy them brand does not really matter however you should be able to install themselves as their are numerous install videos and instructions online.
Tirerack is usually pretty good for everything-brands,price, availability so I would choose what they recommend but it also doesnt hurt to hear about other people's opinions. Personally Ive only heard good things about blizzak's but their are other options.
Rust prevention-Thats debatable, some people dont do anthing while others will rhino-line their entire undercarriage
Driving-Theirs some really good video's on youtube. In a nutshell, after a while it will become very natural to you. However I dont know why you would want good mpg's when you bought an Sti haha.
Detailing-Keep researching this sub-forum and you will find all the info you need
Good luck and enjoy your new suby!