Help in choosing a new calculator

[Ron Ross]

I still suggest the OP consider the Hp 19B calculator as a replacement for the Hp 27s. It has everything the Hp 27s has aside from HEX, BIN, DEC conversions and an entry method that is Chain logic. It also has some units conversions, and it has a better input method for using the solver.
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My wife's favorite calculator is her Hp 27s, but when she took a physics class, she borrowed my Hp 19Bii for the class, because she could enter lots of equations quickly vs the more awkward input of the Hp 42s, 27s, 17b line.
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If the OP doesn't need trig functions, Bob Prosperi's suggestion of the Hp 17B or Bii is an excellent direct replacement although it also lacks combination or permutation functions and as a business calc has algebraic w/o precedence input.