Unfortunately, the best calculator there is for the N800 is a scientific calculator, not a business one --- Free42.
If you don't mind a web-based one, you can try this HP-12C emulator. It barely fits within a full-screen browser on the N800 and is quite usable. You might be able to save the page and Javascript code for offline use. I can't vouch for the precision of this emulator, but the 12C itself is a very popular classic.
I guessed I should have mentioned I'm a HP calculator fan. But if RPN number entry bothers you, then yeah, go for TiEmu.
Note that both TiEmu and Free42 have UIs that are meant for scientific computation, and while they are capable of financial calculations, these features aren't as easily accessible as they would be on a 12C emulator.