I'll try to answer some of these questions. I don't have any pics yet, but I'll get some. I have converted two salters to electric. The first one was a gas motor and the second was hydraulic.
The smaller unit was gas powered, 1.5cu/yd. - (2.5cu/yd with top extensions that I origionally ran - when this unit was on the dually. It is now on a 2500srw, therefore the extensions have been removed). This unit got the same motor that chuck posted - but only 1/3HP. Yes, the origional gas motor was 12HP. Obviously gas engine HP and electric motor HP are not equal. And yes, they run at 1800rpm. It is wired with 8 guage. I don't think that just any solenoid will work because they are not made to stay on for an extended period of time. I run a constant duty solenoid, you can get them right at napa. Yes, it has a 20:1 gear box. I have mine geared with a 12 tooth sprocket on the motor, and 48teeth on the gearbox. It took a few tries to get the gearing correct, and now I have it right where I want it. This unit has been used for 3 seasons now. The 1/3HP has no problem. I have had no corrosion related breakdowns. I have replaced one solonoid and it always gets new chains each season. This unit has performed perfectly since I made it electric.
Total cost - just under $400 bucks.
Throwing the gas engine in the dumpster - priceless.
The second unit I just did at the start of this season. The spreader started out as a 4cu/yd airflow - full hydraulic. It was too wide at the top & too tall to fit in a pickup. I cut it down so that it fits in the dually nicely. It now holds 3cu/yd heaped up. This unit has a 2ft wide conveyer and a 40:1 gearbox. I used the same 1/2hp motor that chuck posted above with the same gearing as on the above spreader. When the spreader is real full, the motor lugs down and spins much slower than it should until she lightens up the load a little. I would like to switch to a 3/4hp motor or gear it down even more due to this. Other than that - it works perfectly, just as the other. It is real nice if you really want to lay the salt heavy, due to the 2ft conveyer. I'm not sure on the spinner gearing,because on this unit I made the spinner belt driven. I really like this unit, very heavy duty compared to the pickup units available.
I'll try to post some pics.
On a side note - I recently took the dually over the scales. She was not quite as full as usual, but close. - 16,500# full of fuel/blizzard on.