You say don't flame you, but people flame because certain topics have been asked repeatedly, some are common sense, and some are at your choice.
What should you replace? Replace whats broke. Fix all maintenance issues before modding.
What should you get? Whatever you want. It's your car.
What should you do to get to get 250-300whp? There are a million threads about this.
How much does it cost to build a KA-T? There are a million threads on here about it. There is also a whole forum for it.
http://www.ka-t.org
The best thing for you to do is to spend a lot of time reading and learning. Go on KA-T.org and read peoples build threads. Go online and look up part costs. The cost of a build depends upon the quality of parts you get, plus how extensive you want to make it. How much fab work you can do yourself will also affect the cost. My KA-T build probably costs me around $8-10K, maybe more. Mine was all out though.
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