Sorry, the link for inductor should have beenhttp://www.mcmelectronics.com/product.asp?product_id=50-3019&catalog_name=MCMProducts
Click on the "crossover components" link at top of page, you'll find lots of parts. You might want a terminal cup too.
Ask your friend if he can design proper volume using the Thiele parameters - they're on the speaker link. That'll add some class to your design and prepare you for an exotic 3 way system down the line....
Midrange is optional. The two speakers suggested have plenty of overlap in their response and those crossover components are around 2500 hz, a high whistle well within both speakers' range. That woofer extends far enough into midrange you don't need a separate midrange speaker. You could spend a lot more money for an exotic woofer with less response but I still advise you to wait until the next speaker project for that. The two speakers linked have same sensitivity, an exotic woofer may not match whatever mid and tweeter you pick and that would complicate your crossover design. Shelving is the term you'd have to add......
If you want to build a three way I'd suggest going for higher end drivers, 15 inch woofer in the $200 each range, 8" midrange and ribbon tweeters. Get a copy of Howard Sams' book "How to build Speaker Enclosures" and digest it, there's considerable science in doing it right. You should be sure you're ready to do justice to that kind of monetary investment. It is SOOO easy for a first time builder to poke a screwdriver hole in your $200 woofer's surround,,, (I know I've done it but fortunately only a $40 woofer).
That little woofer is rated 40 watts continuous so it is conceivable you could burn it out with that 50 watt amp. But it would take enough time that your neighbors would likely have called the police first. SO no, the 50 watt amp would not easily blow them out. You'd have to work at it.
My advice remains start modest. You will learn a lot building these and will be way better prepared to do a fine job on the bigger speakers. You might peruse some speakerbuilder forums...
Did you notice that amplifier kit does NOT include the heatsink pictured? Pay attention to that 1.4 deg/watt, don't buy sinks with a higher number, lower would be better, and be sure to use "heatsink compound" under the power transistors. Bolt them tightly to the heatsink. It is REAL important that the heat get out of your power transistors.. The kit directions should tell whether you need insulators under the power transistors, if so get ones with lowest deg/watt you can. The heat has to make it through the insulator to get to the heatsink.
Material for the amp?
If you want to show off your amp use Plexiglass and put in accent lights - blue LED's or something..
Myself I like wood - your local Home Depot should have birch veneer plywood that you can stain to resemble exotic wood.
There are door kickplates made of brass that'd make a beautiful front panel....... and leave vent holes over the heatsinks.