There are several factors to consider.
The guitar should not be too old.
If the price includes the guitar, there is no point in adding it to another guitar.
Make sure you compare against the same or similar guitar.
If you use a special guitar, like a Telecaster, get the same part.
Take into account the quality of materials or the materials on the guitar and how it will perform.
A guitar used for teaching can also have a price. Make sure that you don’t go overboard or you could end up doing a lot more harm.
Are there any special rules to consider?
This is probably the greatest question you can ask yourself, and it’s one of the most important ones! It is also something that should be answered in the first place: are you serious about it? Are you willing to put a lot of your time and effort into it? Do you have a clear idea of what you want to accomplish? If the answer to both of those is yes (even if at first it seems too good to be true) then this might be the one area where a professional can be useful.
That being said, there are things that one needs to do, and what one needs should be done in a more measured, logical order. For example, you could take an introductory guitar course, but your teacher will only show you one or two important aspects of learning a guitar, and then you would need to make a decision on something you feel will be better for your self and your career. You could also take the beginner course and get an “Introduction to Guitar” and play a few songs with a different guitar and learn some chords and scales. On the other hand, you could take the intermediate course and learn the guitar like crazy, and then at the conclusion of the course you will have mastered all the guitar basics.
The same goes to building a guitar that meets the needs of an instructor (which you need to make sure you do right from the first).
What can one add to this list? Here are some things to consider.
Use the right model at the right time (see the table below for this). This is the main one: when you start, buy the first guitar you are comfortable with.
Avoid the mistake of trying to build a guitar that the instructor is familiar with. This will have the effect of making you sound less serious, and will make that instructor more likely to try to sell