Like most people said keep it standered (though its hard to find one that hasn't been touched, mine has an aftermarket exhaust). The value of valvers have dropped alot lately so its possible to pick one up for alot less than say 6 months ago. Thing with them is what you spend is what you tend to get.
You can lower them if you want. End of the day the torsion car can be raised or lowered, just a matter of you front springs.
Valvers handle very well, so the standerd suspension is fine. Though getting ya hands on a willy widetrack is reccomended to add to it (pick one up from ebay if you can). There is no point in spunking out on expensive 'coilovers' unless your useing the car on a track.
Stripping is also supprisingly effect on them, owners claim there is no loss in handleing but slight power gains, power to wieght ect. That sound deading in the back wieghs abit!
Engine wise, how much money you got? You can slap a 2ltr F7R bottom end and use the 1,8 head. From what i've been told the F7R bottom end is stronger and better if ya gonna start doing head work.
The standerd airbox is fine, howerever a k&N from what i've heard makes the valvers sound lovely, however if you find yeself in traffic the k&n tends to cause heatsoak makeing the car struggle when you move again.
Best bet mate is if you decide to get one is to shop around, and have a good look over the car (Arches! These cars do rust).
Another thing is valvers are strange cars, you get ones that are slow, but you get some freaks that go very well (think someone had a standed valver over 140 odd break on the rollers).
As mentioned clio16valver.co.uk is a good place to get info and see what other people have done, we're a friendly bunch!
oh and phase 1's are the best, and turbines are better than the phase 2s!