I tried upgrading to IOS 11 when it first came out and it messed up my phone. Have all the bugs been worked out? AND will it slow my phone down and be forced to buy a new battery?

iOS 11 works fine for me on my iPhone 6+ and iPad Pro. If you upgrade you will likely find that downgrading is no longer an option. I don't say that to discourage you, but to ensure that you're aware that downgrading iOS isn't normally possible.

See A Message to Our Customers about iPhone Batteries and Performance - Apple.

tt2

Apple has stated that they will be releasing an option to turn off the option to slow down the iPhone in the case of a degraded battery. The iPhone slowing down is because the battery is degraded and doesn't cause or force you to need a new battery. I have iOS 11 running on an iPad mini 2, and iPad Air, two iPhone 7's, two iPhone SE's, and an iPhone X without any problems.