Once you have the basics laid out, it is time to create a list of I/O’s
Analog inputs
Water level
Digital output
Pump on/off
Alarm on/off
So far, this is a very minimal project. any of the basic units could work.
To use shields, the UNO form factor. If you do not need shields, then a mini could work (requires a USB connector board)
water level, leakage, etc are all software. saves a LOT of external parts.
display and data logging with real time clock and such would all be options.
please note that a data logging shield is easier to do if you do not plan to make your own boards. The cost of the shields are darn close to the cost of the parts in many cases.