Basically, I'm thinking about rebuilding my entire computer however I don't know what to do about the power supply, if I was to buy an expensive one what would be the benefits to my computer?
Also are there any power supply's you would recommend?
They can vary a lot, not just the price matters. Some are modular/fixed.
You'd need a good and reliable one to be able to power your components, if you don't have a good enough supply you may see that some devices aren't working properly as there is insufficient power available however isn't that common. Probably best to look at reviews too, I have a Corsair PSU myself and had no problems with it.
The 80(+)/bronze/gold/yellow/blue whatever just means how efficient it produces that power. They all output whatever wattage (i.e 550w) but the amount they pull from the wall (at the socket, from your house) is more than that, the certification is how much efficiency it has in creating that wattage without wasting a fuck ton of power. Stay away from power supplies that companies didn't bother to pant and carefully read reviews from people that have had the power supply longer than the warranty.