For airplane spotting, what minimum specs should a telescope have?
I would not recommend a telescope for viewing aircraft.
As others have pointed out, a telescope that is suitable for astronomy would be incredibly overpowered for viewing aircraft - the aircraft would move out of the viewing frame very quickly, and you would have a difficult time tracking them.

I would suggest picking up a spotting scope (the kind of thing bird watchers and hunters use - good ones can be had in the 50 to 100 dollar price range, and excellent ones abound from about $100 up), or a pair of good binoculars for your plane watching. The zoom factor is far better suited to plane spotting, and either option would be substantially more portable than a full telescope rig.

As for advice on a telescope, there is an Astronomy stack exchange site, and while the entire network generally doesn't do "product recommendations" the Astronomy site's chat might be a good place to ask for advice on picking a telescope. (I'm not sure how active their chat room is, but it's worth a shot!)