waddya wanna know? A 504, rack, PS and I/O ain't cheap (retail about $4K). for a grow room just a simple brick PLC would work fine. (I haven't played with the 504. The 501, 503 & 505, sure a bunch. But its just features being the diff between them.) Rule of thumb is 2 I's for each O.
I even like the programmable relays B&B sells.
How far do you want to go? And A/D? (that gets pricey and sticky) HMI gets costly too.