ive owned subs from all three companies, so I might be helpful in telling my experiences.
The Alpine R's are great street beaters, but the only problem with them is that everyone has them, and if your friend has any sort of individualization in mind, it might not be for him. Also remember, non reconable, not serviceable at all.
The MTX's ive had the experience with all appeared cheaply made to me. Their cones are paper thin, and crack under very slight pressure from anything. I even managed to crack a pair of 9500's myself on just burps....so depending on how rough he treats his equipment, this may be an issue.
The RE"s are great, problem is as you sighted is their new pricing. This might not be an issue if you talk to a dealer near you if you're lucky enough to be positioned near one.
Kickers get loud, there's no doubt about that. They get loud around 45 hz, and that might not be a great street beating target fequency. But at least the cones on them are built pretty tough, and can take some smackn around, and general abuse. Ive heard of people complaing about the surrounds failing or cracking, ive hever had one happen to me so //content.invisioncic.com/y282845/emoticons/imdaman.gif.bc6c552e19aa4ad8c47461144f40eb1a.gif ....one thing I did encounter is lead slap that was cureable with some foam tape on the back of the cone between the leads. Cheap fix's are always a good thing.
May I suggest some other brands beyond the scope of the ones mentioned?
Memphis M3's are awesome bang for the buck, and are self reconeable, which comes in handy for obvioius reasons, and get loud off of realitively mid powered setups. Memphis PR's are agreeably by most a great bang for the buck sub too, allthough not reconable.
http://www.memphiscaraudio.com
Massive Audio CW's are reconeable drivers, get loud, and look very slick. My dealings with them as a company as a whole have been very well, and their subs take a beating beyond the scope of normal abuse.
http://www.massiveaudio.com
Just my input on this, perhaps this might help ya a bit. Good luck homie.