Buffets are all-you-can-eat for everything out there... which means of course, all you can eat veggies and salad!
It doesn't have to be a disaster.
Like others have said, I would suggest one plate of salad to start. Maybe take a small break after the salad to let it digest and fill you up.
Load up your plate with mostly plain veggies (without sauce or cheese) and whatever mostly plain lean meat is available.
I'd avoid foods with unknown sauces and coatings since it's very hard to estimate calories for that stuff. They will likely be laden with salt, oil, fat, etc.
Also maybe you could take a bag of baby carrots, celery sticks, apple slices, etc to munch on in the car before you go?