The amount of priming you do depends on the overall condition of the wood and the paint. Also the type of paint currently on the house could affect the amount of priming. All repaired and sanded areas need priming. If the paint is in good shape then it does not need priming. But, if your home is painted with an oil base paint and you want to use a latex/arylic then the whole house needs priming.
It is difficult to know what is too much. It's my opinion that when in doubt prime it. The cost of primer is very cheap if you find yourself repainting in a couple of years.
You are correct that the primer needs to be applied is a similar manner as the paint. The different brushing directions could show when painted. I recommend rolling the primer and using a brush for the hard to reach areas.
It sounds like you are well on your way to getting ready for painting. I like the idea of starting with a clean surface and your washing has done that. Sanding is also a good idea. If a lot of sanding was necessary then priming the whole house is a good idea.