Just out of curiosity, what year model do you plan on checking out in? I've got about 30 hours in a 1978 182RG, I don't think there's a really huge performance difference between the RG and fixed gear versions so I'll give you my opinions. A lot of this may or may not apply depending on the differences between the plane I've been flying and your 182.
The Constant Speed prop doesn't take as long to learn as you might think. In the 182, you really don't even use the prop control that much. The prop control at full forward gives you about 2400 RPM, and that's the RPM setting you'll takeoff and climb out with. You can even cruise at that 2400 RPM if you want, so in theory you could go the whole flight without messing with the prop control at all. On most flights, the only thing you'll do with the prop control is take a hundred or two RPM out for cruise and make sure you put it up to back up to max RPM before you land.
As you might be aware, the throttle will control the Manifold Pressure in the 182 instead of the RPM like it was in the 172's. The main difference here is that you will need to reduce power slightly when you reach 400-500 ft AGL. The reason for this is that you do not want to keep the manifold pressure at a setting higher than the RPM's for very long, or you risk damaging the engine. However, when taking off from high altitude airports you may find that you cannot obtain a manifold pressure setting higher than 23" or so, in which case you would execute the entire climb at full throttle.
The other thing you'll need to learn to deal with is cowl flaps. These will always need to be open during taxi, takeoff, and climb. You'll close them for descents, landing and anytime you're going to operating at low power settings. You should use the oil temperature gauge and cylinder head temperature gauges (if you have them) as a guide to cowl flap use during cruise. I usually leave them halfway open for cruise.
Handling wise, you'll find it to be more nose heavy than the 172/150. You'll need more back pressure in the flare and might also need more pressure to rotate/climb out before you add in trim. In cruise, it handles very favorably compared to its smaller cousins. The best analogy I can draw is that the 172 feels like you're driving on a dirt road and the 182 feels like you're driving down a paved road. It just has a more stable, less kite-ish feel to it.
I strongly prefer the 182 to the 172 for cross country trips, because the avionics are usually better (a big plus for IFR), it's faster (the RG does 145+ knots), has a longer range (750 NM with the 75 gallon tanks), and has a greater useful load (1200 lbs vs 800 lbs). All in all I'd say it took me about 5-6 hours to become competent enough to fly the 182RG on my own. Without the gear you might be able to do it 4-5 hours or less. I'm sure whoever your renting from has insurance requirements though. Good Luck.
Sorry for the length of this discertation, but I hope it helped.
