What kind of buggy are you looking at? That will determine what kind of auto trans you need to look at.
With a traditional buggy, you'll be looking at a torque converter to replace the clutch on a transaxle like a Fortin or an Albins. The torque converter on a transaxle setup would be ideal for higher hp so you don't have to worry about the reliability of a clutch. The only problem is that the price is steep. Fortin wide 4sp w/ torque converter will run you around 21K. I don't think Albins has come out with their torque converter yet. The torque converter will "cushion" some of the shock loads and help with the longevity of the transaxle too. If you're looking at a transaxle setup, then something like this would work.
If you're going the turbo 400 auto, then the buggy configuration will be more unique...front engine(V8rail, truggy type) or a midengine like barneycar's setup. Also the wheel base of the midengine setup will need to be taken into consideration....length of the trans and rearend going to be longer than transaxle. There's not too many builders that utilize a turbo 350/400 setup for sand buggies so finding one would be more of a challenge.
No matter what, you need to run a HUGE trans cooler. Your auto trans will run hot.
I'm running a V8rail..front engine buggy. I like running an auto....turbo 400 manual valve body with LSx engine. Can upshift and downshift maually. Really easy to drive. Running a low stall so has engine braking and good response. No issues with trans...very reliable.