I love the idea! My sister has a Whippet who suffers in the cold not heat.
Display shield with buttons is not a must have as you could get away with only 2 buttons for this.
Hold both buttons down for program mode, when there you use the same two buttons to adjust the temperature up or down then hold both again to lock it in.
Not good at the programming but you must have a general layout in mind before you can begin to program.
Sit down and write down what you want to have it do from start to finnish (in a flowchart is good).
Eg.
Arduino turns on and does basic test. (Run pump for seconds then fun test display, check no buttons are active and display warning if any faults. Including a current or pressure/flow sensor can detect pump operation).
Retrieve last temerature setting from flash memory.
Check temperature if outside guidelines start pump/if not outside dont start pump go back to checking temerature.
If a button is pressed check buttons to see if both are pressed or if one, which one
Depending on how you want the buttons to work depends on what happens next.
You could say press the Down button then press and hold the Up as well to go to tank temperature set.
Pressing the Up and then press and hold the Down takes you to the bed temperature set.
Or make it easier and add athird button.
Daz