That's a good price on those shocks. Hopefully when I need them I'll come across something similar vs. the $400ish for new ones.
My experience with my Nivomats is that they allow for about 3/4-1" of suspension sag before they start attempting to level out the load. I mention this because if you take out all of the factory rake you'll have a bit of a "cali lean" when you tow. I hate that look so that was for this reason that I went with 2" spacers out back even though I was planing on 1" or 1.5" when I started looking into it. I measured the sag over the course of several weeks with both my boat and camper. When the trailers were first attached the rear would sag and look low but then, after driving around a little, they of course pumped back up and the vehicle once again looked level. Each time I measured I noticed that the rear only came back up to 1" less than the unloaded height though and never returned to exactly the unloaded height.
The other factor is that, unless you're towing with a weight distributing hitch, the tongue weight is going to "lift" the front of your vehicle a little bit. Think of your vehicle as a class 1 lever with the rear axle as the fulcrum and the trailer tongue weight as the downward force. So, under the weight of the trailer, your rear is going to sag an inch or so and the front is going to rise a 1/2" or so depending on the tongue weight. If you start out perfectly level and then attach a trailer your front will be around 1 to 1.5" higher than the rear which is noticeable.
I had 2" of factory rake before. I cranked the front 3" on one side and 2.5" on the other (Due to the apparently normal sag that occurs on the driver's side with the older models with torsion bars) and then did the 2" rear spacers and shock extenders. That left about 1-1.5" of the rake remaining (Depending on the "mood" of the Nivomats. I "removed" around a third to a half of the factory rake) so when I tow my boat or camper or when I load her down with cargo it sits level after the Nivomats "pump up".
Unless you don't mind the low in the rear look when you're towing I'd measure the height difference between your front and rear wheel wells and plan to leave about 1" or so of that when you're done lifting. A 2" spacer + shock extender may be more appropriate since you say you tow often.