I think your approach is reasonable then. An I2C bus that runs around a 10 inch square box should be feasible if you don't try to run the bus too quickly.
If you want to take 20 readings per second from 20 sensors that's 400 readings per second, or one reading every 2.5ms. That's doable over a "slow" I2C bus. The pressure sensors themselves will compute a new reading every 25.5 ms (worst-case, with maximum oversampling) so if you sample sensor 0, then move on to sensor 1, then on to sensor 2, etc. until you reach sensor 20, it'll be 50ms before you get back to sensor 0 and it will have a new reading for you.
Each I2C transaction (i.e., a reading) with the BMP085 involves the transfer of 5 or 6 10-bit words (6 if you want extra precision, so let's go with that). Thus, 60 bits has to fit into 2.5ms, meaning the I2C clock has to be at least 24 kHz, say 30 kHz to be safe. That's entirely doable.
The Rugged Motor Driver: two H-bridges, more power than an L298, fully protected