I definitely think you’ve made a good choice here. If I could by a new Uni right now, this is probably the one I’d get. It looks like your going for unbreakability (kookas, carbon fiber). With your budget in mind, I would probably work it a little bit different:
24" Yuni with Profile - $629
(complete unbreakability in your hub and cranks)
Miyata Base Stiffener Plate - $10
(They say it rivals the CF base and its a lot cheaper - you would have to open up your saddle and put it in, or if you get Unicycle.com to build you an airseat, you can probably get them to put it in there for you - http://www.unicycle.com/shopping/shopexd.asp?id=607)
Kinport Miyata Handle - $19
(Granted, I still haven’t had the pleasure of trying out a reeder handle, but the CNC Miyata handle-clone is my handle of choice. I’ve found mine to be rough, tough, and strong. It looks pretty good and its much more comfortable to use than you might think - much more comfortable than the stock Miyata handle. Just remember, if you want the matching rear bumper, it’s another $19
Definitely stick with the airseat conversion of your choice. I can’t imagine doing MUni without air. You listed $175 for the airseat, but keep in mind either of the Yunis will come with a miyata saddle and I believe Unicycle.com offers an upgrade to air for around $50 (price will vary depending on which seat cover you choose). That way, rather than buying a second saddle, just do an upgrade on what comes with the uni. You may need to talk to the Unicycle.com people on the phone to confirm the airseat upgrade stuff. OR, you could buy the parts and do the upgrade yourself.
I don’t know much about wheels, so I can’t really comment on the wheel upgrade.
With my configuration, you save on the seat base and the handle and dump that money into an unbreakable hub/crankset. I think the handle and plate stiffener are definitely comparable in strength to the parts you listed.
Of course, the numbers don’t completely add up to fit your $650 budget, but these alternate options might be worth thinking about. Heck, you could even stick with the Kookas and switch to the handle and stiffener plate I suggested and really bring down the price.
I hope this isn’t too much crap to wade through. Have fun with your new MUni.