Hi GL. All of the machines that you mentioned are made by Icon Fitness
http://www.iconfitness.com/
You can download .pdf manuals for these models at
http://www.iconservice.com/manuals.html then compare them.
The ProForm C830 has an 18" stride; a distance equal to club models made by
Life Fitness and Precor. ProForm hasn't had such a large machine (model
485e) since about 2000. All other ellipticals found in retail chain stores not only have shorter strides, but have pedals mounted closer to the cranks;
making them feel more like stationary bikes.
I've owned a ProForm 485e in the past. I currently own an Evolution EE120 (stride 16"

, which I plan to stretch to a whopping 30" stride so that it's action is more like jogging.
I've tested the ProForm C830 and NordicTrack CX920 at Sears. Don't worry about flexing of machine joints. Machines are never assembled tight enough in retail stores for the kind of rough and frequent use that they're exposed to. They get used by hundreds of people every week. Your machine at home won't creak, nor walk about on the floor.
The leaf Springs of the current ProForm and NordicTrack models look cool and seem like a logical idea for low impact exercise machines, but actually that feature is superfluous. There is already sufficient elasticity in the column, handle-levers, and pedal-links (if the pedal links are rectangular tubing). The integral leaf springs in the pedal-links impart an unnatural bounceyness to the feel that I'm not crazy about. Ellipticals with well designed motion paths such as machines by Kettler, Smooth, and Life Fitness are built rigidly, because decelerations are gradual. Actually, the ProForm models with 18" stride have an excellent geometry and don't need those springs. My model 485e (18" stride) had amazing low impact without leaf springs. It's crank length, pedal-link length and handle-lever length were approximately identicle to the current model C830.
It appears in the manual that the C830 incorporates some much needed improvements that were lacking in the 485e. It has dual crank bearings now, instead of one. And it appears that those bearings may now be replaceable.
However, I don't think those bearings have seals or a grease fitting, so when they dry out, you better be a mechanic. The crank bearing in the old
485e would begin making noise after about a year. If you continued to use that machine after the bearing became noisy without installing a grease fitting and applying more grease, then that bearing would fail within 2 weeks and you would have to discard the entire machine! At least now you have the provision to replace the failed bearings. I would still recommend installing a grease fitting in the verticle rectangular tubing that houses the crank assembly. If you relubricate those bearings once a year with a grease gun, then they should never need replacement.
Having a close proximity fan directed at you while working out is a great idea, but any fan of diameter less than 12" is ridiculous; even if positioned only 12" away from your body. I've been using a "commercial sized" 16" dia pedestal fan for use with my own workout machines for the past 5 years. I arrange 4 different cardiovascular exercise machines around it and do a circuit training session on all machines; re-aiming the fan by swiveling it slightly to point at the front of each adjacent machine. You can obtain such a fan at Home Depot. They have a nice one by Hampton Bay that sits on the floor without a pedestal, but which is angled upward; just right for use with exercise machines.