You are definitely putting alot of thought into this and I commend you for it. One thing you are not considering with a spotter though, regardless of price/model, is how truly effective it is going to be at glassing at the distances you mentioned. Two reasons come to mind.

One, glassing generally involves scanning wide areas in search of game, etc... The Spotter in question comes with a zoom eyepiece. Zoom eyepieces are notorious for having extremely restricted fields of view. This is definitely going to work against youin terms of your intended use.

In addition to this you also have to consider something called chromatic aberration. This is more commonly referred to as color fringing and results from the different colors in the light spectrum not coming to the same point of focus. It is most commonly seen in roof prism binoculars and spotting scopes (like the XL). At higher magnifications it is more prominent and tends to "muddy" the image resulting in a loss of detail. For an example of it, look at some high contrast object like a telephone pole against a blue or white sky and notice the red or yellow fringe around the object especially along the edges of the image.

The only optics that rectify this issue utilize special glass termed "ED" or "FL" which, by composition, helps to focus all colors of the spectrum to the same point of focus. In terms of a spotter the least expensive model utilizing this that I am aware of is the Pentax 65 mm ED. The scope body alone costs around $420 and an accompanying wide angle ED eyepiece costs around $300.

Second, you have environmental conditions to deal with. Heat shimmer is probably the biggest culprit for inhibiting any optics ability to detect detail at greater distances.

For $400, I would still suggest a good quality 10x42. 10x is probably the practical limit for handheld viewing. If you would buy some type of monopod or finnstick device to steady the binoculars further then you could get away with a 12x42 or higher possibly but then you usually run into issues with overall portability due to size and weight.

Hope this proves helpful.