I would break it down into simpler projects first that will be easier to follow tutorials for.

I'm in college for EE technician so I'm no expert but the resistor values shouldn't have to be exact especially for a 5*5 grid. The closest standard values would likely be fine.

I would break it down into the following mini projects that should take 1-2hrs each tops and you'll be able to combine them all fairly easily. 1. Learn how to use the serial output/input over USB 2. Connect the 5*5 button matrix and get it to output button pressed to serial. 3. Get the LCD to display some text and clar the screen and display new text.
4. Create a simple calculator program that will take input via serial.

Once you've spent a day or so on those you will likely be able to figure out the calculator on your own without a how to guide.

As for power, it looks like the Ada fruit pro has a 5v out. So you should be able to power the board from a battery and the 5*5 matrix from the 5v out.

I would look up some info on the button matrix as well so you know how it works.

Allaboutcircuits.com is a good place to learn any electrical theory you might end up needing. I would look into transistors(BJT) just to have a better idea of how most electronics work.

You will definitely want to build everything on a breadboard first before you decide if you even want to put in a permanent enclosure. I wouldn't if it were me. You're probably not going to end up with a very useful calculator and you'll learn more by taking it apart when you're done and building something else.

I'm not sure what you mean by how to build on a breadboard. But if you explain what you were doing when you tried I should be able to help or point you to a tutorial.

My college is a shit hole and the teachers are not so great SO I've gotten fairly good at teaching myself and my classmates.