price is dependent on many factors. mostly where you buy it or have it built. my buddy usually charges $2 per amp for custom built alts. i'm having an alt built that will put out 160 amps at idle, and over 200 amps at about 2k rpms. there is also other things you can do, like have an external voltage regulator and/or rectifier, cause high amperage in a small case design burns both those pieces up.
install is rather easy. loosen up the tension bolt, and remove the big bolt. then it drops.
1st has one in a delco case. i dunno about anything for custom mount for it, though. mine is being built into a ford case that is rated up to 300 amps. it's a bit larger than my stock one, but my 1.6L has plenty of room for it

you could check with ohio generators or powermaxx. dave cameron, who's on my forum, has a powermaxx on his MSP and said that he's had nothing but problems with the ECU hating not being plugged into an alt. my buddy checked with a place that specializes in converters to use the stock wiring, and they said there was a VERY high chance trying to run an aftermarket alt though the stock wiring harness will fry the ECU.
warranty, well, if ANYTHING happens to the electrical system, then i see the dealer blaming it on the alt. they could say either the aftermarket alt delivered too high of voltage to the component. the stock alt puts out 14.3 volts. mine is going to put out 14.6 volts. and i've thought about adding an external voltage adjustment so i can crank it up to 14.8 or 14.9 volts, but i dunno what the limit for the ECU is, and i don't want to push it.