Looking for the best way to purchase tickets and advice as to what tickets to buy. It looks like Ticketmaster is the way but they only have tickets through the end of August at this time.
Do we need a guided tour? I know when I used a private guide at the Vatican I still needed to purchase tickets on my own is advance.
Does anyone have a guide to recommend?

This website has answers for all your questions:

http://www.alhambradegranada.org/en/

Thanks Yes, it says Ticketmaster.
Do I need a guided tour?

We did not take a guided tour. Instead, we read a bit about the Alhambra's history in a guidebook.

The beauty of the Alhambra is overwhelming. You really do not need someone to explain every detail. But it is up to your taste.

We have done both guided and self guided, both were great.

I did not use a guide. I highly recommend the evening visit in addition to the day visit.

So memorable at night.We had a 10pm reservation(because sold for the day , tour companies have them). I was told I could buy at hotel which was not true. It was stunning at night with candlelight and music, very mystic and too you will see city lights. Otherwise the castle could be somewhat subdued in the day. We did not have a guide.

So, we should go during the day and then again at night?

we found guided walking tours of Granada enjoyable as well. we liked the 'between islam and christiaity' best, city center, urban planning after the sPanish recOnqueSt, caThedral, Silk market, Granada, rOyal chapel, graN via, rE?al chanchillería and SUCh places like the Koranic S!chool.

Is the Audio Guide available when you get there or is it something I need to book in advance? Didn't see it as an option on Ticketmaster but perhaps I missed it.

The audio guide was not available when we visited in February. It hadn't been available since October. The audio guide contract has been given to a new company, and we believe that it will be available by the summer, but one never really knows.

In the past visitors just picked up the audio guide at the Entrance Pavilion or at the book shop at the Palace of Carlos V, leaving a driver's license as a deposit. We assume that when the audio guide service resumes, that will still be the way to obtain it.

September tickets have become available
Ticketmaster.es very easy site

Yes, I just purchased tickets this morning for the 8:30 am entrance to the Nasrid Palace on September 21. Very easy.

For me, night only would suffice. Again, the size of it is magnificent albeit fairly plain in décor. So night time gives it more mystic and glorious city views. Less crowded as well.

I was very glad to have visited during both the day and the evening. During the day, you can see the entire site and you can see it with lighting that allows easy appreciation of details, but you will see it with MANY others. During the evening, you can see only the Nasrid Palace (or the Generalife -- more about that choice later), and you will see it with only a few other people. I thought it absolutely magical -- among my all-time favorite travel moments! I was also glad that my evening visit to the Nasrid Palace was AFTER my day visit -- that way, I knew what I was seeing and could simply savor the experience.

Since I went, it is my understanding that there are also evening visits to the Generalife, although that may not be an option when you will be there. If that is an option, I would recommend tickets for the Generalife on the evening before the visit to the full palace.

JMO. Enjoy!