Sushi is totally fine for you depending on what you get. Since you haven't had it before, I doubt that raw fish is going to be particularly appetizing for you....
here's what I would recommend for a buffet style restaurant:
(1) Salads- they have a lot and they're good and lightly dressed in vingarette based dressings so it's healthy.
(2) Miso Soup- some tofu, broth, and veggies never hurt anyone. add low sodium soy sauce for flavor (green cap typically)
(3) NO california rolls or baked rolls of any kind! They are high in carbs and not very fresh.
(4) if you can bear the thought of raw fish, go for sashimi. particularly look for salmon or hamachi. They're a little easier on the palate
(5) if raw fish is totally out of the question, get some terriyaki chicken lightly dressed. it's really the sauces that are fatty, so if it's already dressed just remember portion control
(6) Tempura fried stuff isn't that bad for you either- it's a different way of frying, and it's usually done to veggies so in a way it's "healthier." If you choose this option, remember portion control (like 4 pieces of the greenier veggies vs. all squash or sweet potato).
Hope that helps!
Sushi is very healthy, very low fat. It is mostly rice with a bit fish and algae.
As long as you dont overeat, it is actually considered a good food to loose weight.