My perspective comes from having bought a pre-built rear hub bike (Magnum Peak), and then converting a bike I already own (Trek DS 8.4). I bought the Luna BBSHD mid kit with a 48v 13.5 AH battery...it was under $1500 shipped.

While the Magnum Peak is nice, I LOVE the way the Trek rides. I toned down the power some and it's as smooth as silk. For me, the knowledge of what it took to build the bike (it really was easy even though I'm not a bike mechanic) and the ability to reprogram the performance anytime make the Trek build even better. If I had built the Trek first, I'd never have bought the Peak. I've ridden the Trek over 1600 miles since I converted it in late April! I have 850 miles on the Peak, though it's a month older -- most of those miles came before I converted the Trek.

A few notes that may be of interest:
1) If you still like your current bike, you know you'll like the ride after you convert it.
2) You can probably go with somewhat wider tires if you want to. My Trek had 700c/38s, and I've fitted 29/2.2 Serfas MTBs (on the same rims).
3) You can also look at batteries from somewhere else. I decided I wanted more battery power, so I bought another Shark-sized battery on Alibaba -- 17AH for $392 delivered. I know you have to be careful buying stuff from China, but if you look closely, you can buy exactly the same product that you find here and save $150 or so. (My favorite ride is 43 miles with a 1600 ft elevation gain in the middle. I usually average around 18 mph. That uses about 60% of my 17AH battery.)
4) I had never heard of jack nuts, but they're the cat's meow for installing a connection on your frame for the battery mount (unless you're wanting to hang it in a bag or put it on a rack or something).

One last note -- whichever way you go, you'll love your e-bike! I used to think a 20-mile ride was about all I wanted to do...now if I ride less than 40 miles, I feel like I'm not done...you know, like playing 9 holes of golf, or bowling one game.