You've just gotta decide what works best for you. If you try to search for what's best you will never end up anywhere.
1. p10's are great, but do take a little more learning/setup to get going. need to make a waterproof case for them
2. square or bullet, doesn't matter. the lights are the same
3. voltage, 5 or 12. again, doesn't matter too much unless you are going to run really long strings and don't want to inject too often. they are both so widely available in so many different types. I do suggest picking one and sticking with that throughout to make it easier to have spare pixels/power supplies on hand
4. you have to inject if the strings are long enough. if they aren't you don't need it. your setup will dictate
5. 180,270,360. look at your yard and decide. if you do a 360 will people be able to see the backside or no? if your house is in the middle of the block and people can only see from the front a 180 is enough. maybe 2 180's rather than 1 360
6. why not both?

its all up to you. and you will probably keep changing and adding stuff throughout the year.