The DIY sub comes with all you need inlcuding driver, cabinet and amplifier, it's just a matter of screwing it all together, you are saving money, because there is no marketing, no assembly charges and of course no tax on the net. So you can easily build a 200.00 sub that would match subs costing 2-4X more in stores.
Speakers are subjective, what you may like I may hate, so I can't comment on the Wharfdales you mention. I would prefer buying the 450's not only because they are great speakers but come with a warranty. But don't let that dissuade you, if you find speakers that sound great to you at a good price snatch em up.
When talking reciveers, i was thinking of last years models on refurb Shoponkyo and Accessores4less always have the 605,705 and 805 on sale for a great price. But if the Yammie has the power and features you need, then buy it.