I have the 18" WSM at my camper and use when we go to the lake. I have owned this unit for quite some time I'm thinking some where around 12 years. It works just as good now as it did new. I have had no issues with rust. It gets well cleaned every year when we winterize the camper, and I coat it with a thin coat of oil to prevent rust during storage.
I'll try and answer your questions.
1. Will an 18"er hold a turkey?
I have smoked 18+ pound turkeys in it no problem.
2. Are there any modifications needed?
The mods I did were;
1. Replaced the water pan with a Brinkmann charcoal pan, it holds almost 2 gallons of water. HIGHLY RECOMMENDED
2. Added 2 thermometers; 1 in the lid on the other side of handle opposite the vent, and 1 in the middle section just above the grate opposite the door. NOTE: There is not allot of temp difference between the 2 grates somewhere around 20 degrees if I remember correctly. So in hindsight 1 thermometer in the lid would be sufficient in my opinion. However 2 make it look really cool.
3. Added a 3rd cooking grate in between the original 2.
4. Added handles to mid section. HIGHLY RECOMMENDED
5. Added a grate that fit inside charcoal ring. This grate sits 90 degrees to original grate creating small grid work allowing charcoal to totally burn up and not fall threw the grate. Recommend
6. Added 2 long U bolts threw the charcoal grates. This allows me to remove the grate if needed, and aids in shaking coals to let ash fall threw, as well as holds the charcoal ring in place. HIGHLY RECOMMENDED
7. Replaced door latch with longer wider piece. This helps hold the door in place, I use the door upside down, seams to fit better.
3. Does it come with a temp gage or a warm/medium/hot gage?
No mine did not I added them. The newer ones may I do not know.
4. To be clear I can adjust it to the temperature I want and it will maintain?
Yes, to a point. I have to adjust the air vents every so often to maintain temps, its not a constant adjusting by any means. As you use it you will learn when and how often you need to adjust. I might adjust them around 4 times in an 8 hour smoke. I shake the ash off the coals before I adjust the vents, and more times then not this fixes the lower temp.
5. My Brinkman eats charcoal, how is this one?
I have done 18+ hour smokes on it using 1 load of charcoal using the minion method, and still had coals left. That's some where around 1/2 to 3/4 a 20 pound bag of charcoal.
Here is a link to a very useful page on the WSM. http://www.virtualweberbullet.com/tipsfaq.html Hope this helps you out.