Have you tried calling the AC manufacturer, confirming it’s under warranty and asking them for a list of recommended dealers?    

I'd recommend the latter for someone starting out-- you could use a 203k loan which helps you finance rehab costs into your loan.

Does anyone have recommendations on their experiences with a particular brand?

I HIGHLY RECOMMEND HAVING A GC WALK the property with you...if the thought is even crossing your mind then the answer is yes.Up here in Chicago we charge a fee of $197 to walk properties a provide a page report with ball park estimation.With contractors especially you get what you pay for, so find SOMEONE WHO KNOWS THEIR TIME IS WORTH SOMETHING! 

Is it recommended to have a LLC and who can help creating a LLC (tax consultant or lawyer)? 

If your certain you are doing this I would recommend just doing a cash out on your properties, around mid 7's.. having the funds available and put them into a savings account earning 4-5%..

Does anyone recommend investing in this area?

Do you have any other recommendations for me? 

@Joe WoodI agree with @Brian Eastman, the taxes from a $100K distribution would be high.

If you're going to use national average data, I would recommend using all of the other national data to provide as a benchmark.