I know this is old but figured I would chime in.
BDS from what I've seen is middle of the road stuff. Good for gaining lift, but nothing groundbreaking. I had a BDS leveling kit on my 2007.
Lifting and maintaining capacities seems hit or miss. As long as it's just blocks in the rear you shouldn't lose much except for the higher CG and all that comes with that when towing. 6" lift will severely impact goose neck or fifth wheel towing cause you bring contact with the bed and trailer into play. That's the only reason my daily driver only has a leveling kit in the front. If I lifted the rear id never get under my fifth wheel.
Most trucks already have blocks in the rear at factory height so you won't lose much if you go with a simple kit and re use your factory springs.
Just pick a kit in your price range with rear blocks and you're done. 6" lift will probably have new control arms and new coils or possibly just spacers. I have a 4" kit I bought but like I said I couldn't get under my trailer if I installed it. It was just blocks, spacers, control arms, and hardware cost around $450 I think.
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