Hi willy45,
I too am in the process of building a weather station.
Uno will be fine, or Arduino mini/micro/nano, which will fit on a breadboard, which will be a convenient way to prototype your solution.
For barometer, I would recommend a bmp180 on a breakout board. For humidity, an sht21 (again, on a breakout board). For the real time clock, a ds3231 (guess what, also on a breakout board, which will also have a backup battery). All 3 of those use what's called the i2c bus to transfer data to the Arduino, so only 2 pins needed for all 3 of those devices. In addition, all 3 devices have built-in temperature sensors, so you probably won't need the existing indoor sensor.
Paul