Custom Number Format: Resistor Values

The closest i can get is this.. (I'm using those basic functions only)

Put the input in your A1

then

=IF(LEN(A1)<=3,CONCATENATE(A1,"R"),IF(AND(LEN(A1)>=4,LEN(A1)<=6),CONCATENATE(LEFT(A1,LEN(A1)-3),"K",MID(A1,LEN(A1)-2,1)),IF(LEN(A1)>6,CONCATENATE(LEFT(A1,LEN(A1)-6),"M",MID(A1,LEN(A1)-5,1)),"??")))

But you still need to fill those cell with "??", so yeah, not a perfect solution I guess.

For the number-to-string conversion, maybe it's a good idea to make two columns for both the number (120) and the string (120R), and you can just hide/unhide the whole column if you want to print the page or something

Hope it helps