Mars Exulte

The high detail model you see up close to an aircraft is not used uniformly at all distances. It actually switches to progessively simpler models as the object is further away, lessening the burden on your PC without discernable loss of detail. This is called LoD (level of detail) and typically there are 5+ models it switches between.
 Aircraft disappearing in between two points of visibility (you can see it zoomed out, can see it zoomed in, but not at a halfway point) is typically due to poor or outright missing LoDs. None of your settings are going to have an effect on something that is literally NOT THERE and there's not really anything you personally can do about it.
 It affects a number of units. In particular the MiG-21 used to vanish at intermediate ranges as I remember but I THINK they fixed that a while back, but don't take my word for it.

#1 Compression
  Files are heavily compressed for download, so it's faster, and then extracted afterward.
#2 Patching files
 You may down 150MB of ''patches'' that must then apply to several hundred files totaling several gigabytes of rewrites. Basically, it's ''appending'' to a preexisting file, rather than replacing it outright.
#3 Replacement vs addition
 You may download 25GB, but a lot of that will replace stuff you already have, rather than adding to it, so your actual total install may only increase 5GB, or not at all, or technically could even shrink if the new files are smaller.
 
End result, download size is not directly related to the storage space required.
 
 

Generally, it should carry over for the most part. It will depend on the specific plane you're using and how much it has or hasn't been changed. If your primary plane was one that was in dev and likely received substantial updates, you'll probably have to redo your controls.
 
The export LUAs for displays should all work, though, I'd think. You'll just have to try it and see. There's plenty of folks around that can help you make a new one if needed.