1. The Spotting Scope
The basics have not changed when selecting a spotting scope for digiscoping. Keep one digiscoping principle foremost in your mind - the main factor, by far, in picture quality when digiscoping is the optical quality of the spotting scope. The camera is actually a distant second in this regard, so put your money into the spotting scope, first. Digiscoping is no different than any other type of photography when it comes to picture quality - it's all about the camera lens and, in digiscoping, the spotting scope is your camera lens.
What makes a good spotting scope for digiscoping? The best feature to look for in a spotting scope for digiscoping is a high grade objective lens with special, high performance optical glass - ED, HD, XD, APO, fluorite and so on. Just as with camera lenses, an ED or other high performance glass objective will produce the sharpest pics with the best color and resolution. These special glass versions of spotting scopes produce the best images at high magnifications, visually, and at ALL magnifications, photographically. Yes, special glass objective spotting scopes are expensive but they are a digiscopers dream. For digiscoping, it is better to choose a small spotting scope with an ED or other special glass objective than a large spotting scope with a standard glass objective.
Next consideration in a spotting scope for digiscoping is the spotting scope eyepiece. Remember, the eyepiece in a spotting scope is half of the optical system. The eyepiece is an important consideration for both visual use and for digiscoping. For visual use, a zoom spotting eyepiece is the most popular and versatile eyepiece choice and, yes, a zoom eyepiece can be used for digiscoping. The best eyepieces for digiscoping, though, are fixed power, wide angle eyepieces with long eye relief. Why? Fixed power, wide-angle spotting scope eyepieces usually have wider lens elements than zoom eyepieces and fixed power eyepieces typically offer more eye relief than zoom eyepieces. This makes camera placement behind the eyepiece less critical with a fixed power eyepiece, but, more importantly, it also means a fixed power eyepiece produces less vignetting (darkening at the edge of the frame in your pic) than a typical zoom eyepiece. Indeed, some zoom eyepieces, especially cheap ones, produce so much vignetting that we end up with a round frame pic, rather than the usual rectangular pic we are used to seeing on our camera. This is sometimes called "portholing", since it is similar to viewing the world through the porthole of a ship.
The bad news is that you typically don't get the option of fixed power eyepieces on lower and mid-priced spotting scopes. To get the option of fixed power, wide angle eyepieces, you usually have to move up to more expensive spotting scopes. Really no choice, though, if you want this fixed power eyepiece option. With very few exceptions, spotting scope eyepieces are not interchangeable between brands or even between models within the same brand. The good news is that you can always crop any pic, even a porthole shaped pic, in your computer software, once you download the pic into your computer, but getting a full frame pic in the first place gives you much more flexibility when editing.. (Hint: when choosing a fixed power eyepiece, keep magnification around 20x or 30x. When you factor in the optical zoom on the camera with these magnifications, you have all the magnification you will ever need.)
Okay, you don't have the budget for a high grade spotting scope. Now what? With spotting scopes, optical quality is very directly related to price, so buy the best spotting scope you can afford. You don't need a lot of magnification or even a large objective lens, just the best optical quality and that will be reflected in the price. If a small scope and a large scope are priced the same, which do you think will have the best optical quality? Right, the smaller scope at the same price. If you cannot afford a model that offers fixed power eyepieces as an option, you are stuck with a zoom eyepiece, but not all zoom eyepieces are created equal for digiscoping. For digiscoping, at least try to find a model with 15mm of eye relief or more. That will produce less vignetting and portholing in your pics.