First off, welcome to the wonderfull world of autos!
I built the 2004 Japan winner a while ago, so here goes:
If you're uncomfortable with TS50 and want an a/b version, check out Hiroboy's Zero range of paints. He now has a Mica blue - and an entire range of Subaru "real car" colours, if you want to do the normal kit and not one of the WRC ones. (There's not a Tamiya bottled equivalent) I've used TS50, so cannot comment on that specific one, but the rest of his paints that I have used are awesome. If you do, in the end go for TS50, use it either over a white primer - grey primer will not do the colour justice. And build it up very gradually.
I use TS13 mostly. It does tend to be a bit harsh-ish, but I've almost never had problems. There is a zero clear as well, if you're going that route, btw. But it's one of these 2K's you have to mix. It (TS13) works fine as long as you (a) don't flood it, otherwise it will damage the decals (mist coat(s) first, then wet) and (b) don't have any of your decal solvent still trapped under the decals - that will pretty much screw it up. But the latter is true for most clears.
Decals works perfectly with microset and sol. Weathering does work very nicely on a rally car, done it myself (on a Focus) for an age-old Rally GB. Monte is a tar rally, though, so there's not all that much to weather as in a gravel rally.
The kit itself does not have problem areas. Though a different version, I can't imagine them having gone "backward" with the later release. Engine detail... which engine detail?! There's just some parts resembling the bottom of the engine (hood is closed) and that gets covered by the protection plates anyways. In some they are clear, so you can leave it that way if you want to, can't remember whether mine was or not.
Anything else, just shout.
Oh, and if you want tons of input and the real experts to speak to (I am by no means an expert) why don't you join this build up in our race car GB?!