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I would replace everything that generally could wear out or weaken with age: Definitely the tensioner, idler pulley, and the water pump. The pump is cheap to do now, it will be much more expensive to do the water pump separately in just 5k miles...you never know. Imagine the frustration of having to do the same work again to get to it. Not to mention cost, if you have to have a shop do it.
Pay close attention to the timing marks of course. I dont' have a manual handy for details, but I would recommend getting one to see if there is a certain procedure for doublechecking the belt... like turn the engine over two full rotations and verify that all the marks are still lined up right. (taking up all the slack in the belt)
Here are a couple of diagrams that may help you too
And here's a blog article covering the V6 mitsu timing belt.. it's a little generic, since the same engine is used in so many other cars. But all in all, yes the belt can be done without pulling the engine :) You may need to remove some junk out of the way, like the radiator and fan assembly.
I haven't done the seals and I don't have a manual available to me right now, so I honestly don't know for sure. Sorry. I can try to find out later today but right now I have to leave. Not sure when I'll be back.