Hello John L, and welcome to the forum.
First of all, no apologies necessary for your questions. I am probably most guilty of asking too many, but I will say it here yet again: The fellow members of this forum are exceedingly kind and informative. You will hopefully get many responses with some opinions differing from mine, and I would listen heartfully to all of them. We're all here to help you make the best choice.
Here are some websites that may help you:
www.iwata-medea.com
www.dixieart.com
The Iwata site can lead you to a chart showing the different applications for all their models, and pictures and details of each model of airbrush, but I have found it a bit hard to directly compare things such as spray widths, but at least you can see them. This alone is the very reason that I bought the extrra needle/nozzle. I was hoping to cover my bases on some of my larger model aircraft.
The dixieart site is for the supplier that I bought from. Their are others that other forum members recommend also. I can't remeber exactly their names, but again, a search will yield the posts with that info.
Also, you can do a search of this forum using keywords such as Iwata, Eclipse, Revoution, Bottle feed, gravity feed, etc, and find all past posts on this very subject, and I will tell you, there are many, but all worth looking at.
I own an Iwata HP-CS and I think I can answer a couple of your questions.
First, the difference between Revolution and Eclipse is that the former is a single action and the latter is a double action. What this means is that with a single action, you simply pull back on the trigger and get both an airflow and a paint flow. With a double action, you first press downward on the trigger to get an airflow, and then while still pressing down, pull back to get a graduated paint flow. You will still get a graduated flow with the single action, but when mastered, the double action will give you more controlled results.
The Revolution is marketed for the beginner airbrusher/hobbyist, but the double action is not difficult to master with a bit of practice.
Yes, you can buy and interchange different size needles and nozzles. In fact, when I puchased my Eclipse HP-CS I also bought an additional .5 needle and nozzle. The reason I did this was because no one could say for sure what the max width spray pattern would be with the .35. I haven't used the .5 yet, so I can't tell you from experience, sorry, but maybe someone else on this forum can.
Since I model aircraft, I can't say much about its suitability for car models, especially 1:18th scale.
As for the cup size, I went with the 1/3 oz. because I didn't want to keep refilling, and I'm glad that I did. The larger cup has never gotten in the way of seeing my paint line, even when painting tiny Camo lines. If you are really concerned about the quantity of paint for your cars however, Iwata makes basically the same brush with a bottom feed bottle, but there are some advantages to having a gravity feed, such as better paint flow through the brush. This doesn't mean that the bottle feed is bad, though, just different.
I took a lot of time trying to decide what to buy, and I'm glad I did, especially after getting input from this forum.
You may indeed need something different for your car modelling needs, but I love my HP-CS.
Check out the other posts, and good luck. Let us know what you decide on.
Stinger