My current problem is knowing the best way to control the 7 and maybe more fans that I will be controlling. This fans are currently set in 5 groups or channels based on location and purpose in the case so. I want to be able to set them manually and also set them programmatically.
I question the need to control the fans independently... i.e. It wouldn't make sense to have some fans running full-speed and others off.
So I was thinking that the best way to do that would be to have a slide pot for manual input
That would be OK. But it's mechanically easier to drill a round hole and mount a rotary pot. Another option would be to have buttons for "faster" and "slower". And, with an LCD you have the option of "soft buttons" where a button can have one or more functions, controlled by software.
then have digital pots sending a signal to a mosfet to then be the control on the 12V line to drive the PC fans.
It's better to use PWM to a MOSFET. You wouldn't need the digital pots and you already have PWM outputs. Linearly controlling the voltage/speed with a pot (or digital pot) is inefficient and it requires the MOSFET to dissipate heat.
Also, I prefer to buy all my parts in basically one go. I know this probably is not the best practice, but with limited budget, shipping costs become a pain.
It's not "bad practice", and I usually like to buy a few extra parts in case something "goes wrong". But realistically, you'll probably end-up placing at least one more order before the project is done.