Since you don't need a lot of I/O, a Uno would be more than sufficient. If space is really constrained there are smaller versions like the Nano, but if you aren't very experienced with Arduinos I'd recommend sticking with the Uno.
How much accuracy and precision do you need to measure the voltage on the +25 VDC lines? If you just need a rough idea, you could use a voltage divider on each line, e.g. droping the sensed voltage by a factor of between 5 and 10, and connect the resulting lower voltage into three separate analog input pins on an Arduino.