How many strands is adequate? I have calm older horses only
If the electric is going to be your only fencing then one strand is not enough. If for no other reason, it will break at some point and then you have no fence at all.
I looked at it this way, you want your fence to be a visual barrier as much as the electric shock part being the barrier for those times when the charger is not working, etc.
Two strands would work for an older horse. I suggest three (Minimum).
Is it better to use plastic fence posts or metal with insulators or wood with insulators? Plastic would be way more economical.
Best? I would say good quality closely spaced wood posts with insulators.
Step in plastic posts or metal t-posts will work too. If you use metal t-posts be sure to use post caps. When you thread the fence through the insulators (or plastic posts) do you have to stretch it like barbed wire?
How the fencing is installed depends on what type of fencing you are using.
Follow the manufacturers recommendations.
Fencing like electric tape needs to be taught but not tightened where fencing like plastic coated high tensile does need to be tightened (so it would not work with step in posts). How does one buy a fence charger? Is solar the best? Electric? Or batter powered? What do I need to know before I buy one. They range in price for like 60 bucks to over 300 bucks.
Solar chargers are great if you do not have electricity available. Past that they really lack the ability to give a good shock. They give one shock and if the horse touches again right away there is no real kick to the shock. Your best bet is the type that you plug into an outlet. The size will depend on how much fencing and what type of fencing. Anything else I should know before I tackle this project?
Grounding. Having your system properly installed makes all the difference in how well it works. Read the installation instructions carefully. Many manufacturers have really nice instructions on just how to make sure your fence is installed correctly. Look on line for those. Sometimes there are even free videos at the store.