You are correct that the Cosmos GT is essentially the same as the Astro Fi. Here is a brief description of the Astro Fi from my book, The NexStar User's Guide II:
The Astro Fi series is an introductory-level model similar in physical design to the
SLT. The tripod is constructed primarily of aluminum and is not quite as stable as
the steel-legged model provided with the SLT; be sure all the bolts are tight to
improve stability somewhat.
The mount features a standard CG-5/Vixen dovetail that allows the use of a wide
variety of small optical tubes; the weight capacity is about 8 pounds/3.6 kg. The
supplied optical tubes come with a red-dot unity finder on a quick-release bracket.
Power is provided by 8 AA cell batteries in the provided power pack or by another
12 V DC external power. The provided power pack must be placed on the tripod tray
or hung from an opportune location on the mount or tripod. The best solution is to
utilize hook and loop (Velcro) straps to affix it to the fork arm (Fig. 3.9).
The unique feature of the Astro Fi series is its built-in WiFi to make use of the
Celestron SkyPortal app on an Apple or Android tablet or smartphone. The
SkyPortal app takes the place of the physical NexStar computerized hand control.
In fact, the Astro Fi, unlike all other models discussed here, does not include a hand
control. It is, however, compatible with the standard NexStar hand control as well
as the StarSense AutoAlign hand control and camera. That said, one who purchases
the Astro Fi can bypass the cost of the hand control by making use of an existing
tablet or smartphone. A word of caution: before buying an Astro Fi, be sure to test
the SkyPortal app (available for free in Google Play and the Apple App Store) on
your smartphone or tablet to ensure it is up to the task of running this large app.
The Astro Fi series includes inexpensive Kellner eyepieces, and the refractor
models come with a low-quality mirror diagonal. The optical tubes in the Astro Fi
series can provide much better views with improved eyepieces and diagonal.
The only significant differences are that it only has a single AUX port and it has an older WiFi module that has lower power and may experience drop-outs (disconnects) from your smartphone/tablet. The first Evolution mounts had that same WiFi module but the Evolution suffers more from it as the module in the base and obstructed more by metal. In the Cosmos GT it is on the side of the fork arm and suffers less obstruction.
Regarding autoguiding - this is only possible on NexStar/Cosmos mounts if they are mounted on a wedge and polar aligned. There are two notable options for the Cosmos mounts. Mederos CNC makes a wedge for these mounts (http://www.ebay.com/...eros_cnc/m.html). Additionally, it is possible to mount these scopes on the NexStar 4SE/5SE tilt-head tripod. Note that the 4SE/5SE tripod has no provisions for fine adjustments so polar alignment is very difficult.
THAT SAID - the gears in the GT/SLT/Cosmos mounts are low quality with a LOT of backlash and do not lend themselves to accurate autoguiding so personally, I wouldn't spend much money on that. Instead, spend time perfecting the art of capturing 15-25 second exposures with your camera and then stacking them to increase detail and reduce noise.
Regarding stability, the fork arm and motors are quite stable up to that 8 pound limit. It is the tripod that shakes. Be sure to tighten all the bolts and only extend the legs are far as necessary to maximize stability. I did replace the tripod on my NexStar 102 SLT with a NexStar 4SE/5SE tripod as shown in this image. I don't actually use the tilt-head for EQ alignment, but the tripod itself is much more stable. Unfortunately, the tripod isn't sold as a separate item so about the only way to get one is on the used market. Note that the original NexStar 4GT and Tasco StarGuide 4 both used this same tripod so your search could include those models as well.
The motor you linked to on Celestron's website is what is already in the mount. Again, the motors are sturdy, but the gearing is a bit rough. The gears will last for many years but they don't provide ultra smooth movement as you would want for longer exposures. There are no after market upgrades to change this.
Author of "The NexStar Users Guide II"
Author of "The NexStar Users Guide"
Author of "NexStar Observer List"