'13 Ram 2500 - how much lift and pull 5th wheel?

I'm pretty tall with my ram, I'd decide on a 5er first and the figure out your suspension mods...after the hitch is in and trailer hooked up.

Get the 5th wheel first, then think about modifying your truck. You don't want to spend money on moding your truck only to find you need to undo it because the 5th is too nose high.

Keep your truck as a toy and have fun with it.

With a stock Ram you'll be hard pressed to have most 5th wheels level when hitched up. JMO and it's what I did would be to get just a leveling kit installed. 2" springs will make it look level. You should have about 3" difference between the front and rear fender heights. After you hitch up the 5er the rear will drop about 2+". You will need air bags to re-level the truck.
I have a 2500 and if you have a Cummins in yours then the payload is fairly low. My 5er's pin is only 1800lbs. That and the gear we put in the back of the truck drops the rear about 2". That makes the headlights shine upwards. I added airbags to get the rear up a bit. With that being said my 5er runs nse high about 1-1/2" or so. Before the level kit was added the truck and 5er were darn near perfectly level with no bags, just the stock suspension.
It still tows just fine but when staying for a quick one nighter I need to unhitch or the 5er's too high to be comfortable in.
Not all 5ers are created equal so some premeasuring will help.
If you do have the Cummins then you'll be limited to lighter pin weight 5th wheels. Usually mid profile. Those tend to not be as high.
A little research is key.

The Rams stock height is already too high for most 5th wheels. Most people are having to raise the fiver as it is.

Newer trucks in a lot of cases are already at the height limits of a lot of fifth wheels. Lifting is only going to make things harder, or worse. When trying to achieve the recomended 6 inch clearence between fiver and truck many owners of newer trucks have actually resorted to lowering the truck.

Well, we get a ton of people asking to how to level their 5th wheel, on new stock height 4X4 TV, stick to the slightly larger tires and be happy.