My experience with Sigma has been very positive. I would suggest you determine a list of focal length primes or zooms you wish to consider, then do some searching for reviews and opinions for individual lenses. There are many superb non Sony lenses available. Once you have arrived at a want list, then I personally would buy used on Ebay (where you can get a full refund, even if the seller won't accept returns)
Remember that many of the non native lenses will be manual focus, that auto focus with non sony lenses can be problematic...so you have to do some research.