Welcome Arabella.
Mechanically speaking, the 99 is one of Singer's simplest and most rhobust designs. Very little to go wrong on it, these machines will be going clickety-click for a very long time.
Also, the metal finishing in the 30s is of a better standard, if only they applied the luxurious decals of the earlier 28/128K.
I find hand cranks a joy to use, makes one wonder why modern machines are stiff to turn by hand.