It doesn’t matter which calculator you buy, as long as its one among the allowed ones. Buy the plain HP12C if you can, if it’s allowed by any import restrictions you might have. It’s allowed on the exam, it’s OK, it’s robust and it’s been around for 25 years, it’s sold in many places around the world. Don’t worry over RPN, it’s by no means more difficult to learn how to handle it than learning the curriculum itself. The choice of calculator will probably not swing your exam results either way.
just remember that the HP12c does not give the number of periods as a fraction. If you are solving for number of periods in TVM. it only give the answer as a rounded up whole number. It says in the stalla readings that the CFAI is aware of this and will adjust the exams.