I like the Texas Instrument TI-30 line of scientific calculators. The have all the functions you will need and they are easy to use. The typical price range is around $10.00 plus or minus a buck or two. Get the battery powered calculator, and if you have been using it for several months, replace the batteries before the examination. Solar powered calculators need enough light to operate and while every attempt is made to make sure there is adequate lighting at the test site, who needs to take that chance on the day of the examination?
I don't suggest buying a new calculator that you are unfamiliar with the evening before the examination. You may waste time looking for function keys which may make a difference in the niumber of questions you are able to answer. Remember, you only have so many minutes to answer the questions correctly.
Good luck - Al