should I buy a 100k digital pot since it can pretty much act like any pot from 1k-100k ?
In general, you can't make one value pot "act like" a different value pot.Sometimes the value in't critical, and sometimes it is...
If you connect a pot between 5V and ground, with the slider going to the Arduino's analog input, almost any value pot will do the same thing... In the mid position, you get 2.5V out. If the pot value is low, you have more current though the pot, (and more power dissipation). If the pot value is high, you can get noise pick-up and interaction with the high-impedance Arduino input and you might not get 2.5V.
In other situations, the circuit could get totally fouled-up if the pot value is off by a factor of 5 or 10.
An important part of engineering (or modification/repair) is knowing when a component value is critical and when it's not. That's not always an easy thing, because you have to understand the circuit it's used in. Somebody might say, "Just use a resistor somewhere between 10 K and 100K", or "Use resistor around 10K." Other times, somebody might say. "Use a 330 Ohm resistor", or if it's more critical, "Use a 330 Ohm 1% resistor."
If you are working with simple voltage dividers & resistor circuits, then an understanding of Ohm's Law
and Kirchhoff's Laws
may be all you need to know. In other cases you may need to understand AC circuits, digital logic circuits, op-amps, transistors, MOSFETS, etc.