I've never heard of hydraulic steering being affected by cooler temps. I've had hydraulic steering on my boats for years, and have stored them / used them in below freezing weather with no problems at all.
What else is there to know about hydraulic steering? Well besides it being the absolute best steering system available for your boat, there are very few things to be aware of that come to mind right now.
1) make sure your helm resevoir is full to manufacturers specs - but don't overfill it... because (go to #2)
2) hydraulic fluid will disolve the glue in your carpet if you spill it / overfill your resevoir.
3) make sure you "bleed" the system correctly and completely when first installing it (just like you do on your car brake system).
I've installed systems on boats before, and they are fairly straighforward to do. It is more work removing the old cable steering from the boat than it was to install the hydraulics. I'd highly recommend them to anyone thinking about upgrading their steering. If you have any specific questions about installation / etc... let me know.
All the best,
Glenn