Depends on how many trailers you have that will be needing ice.
A few things to consider
The storage bins under the ice machines are insulated, not refrigerated. That means ice is always melting.
If you have an ice machine unplugged for a couple days and the machine and bin are in a truck the bin will almost be empty or will be empty in that time. So when you start the ice machine up you will have at least 1 to 1/2 days until your bin fills up. Remember the 300 or 400 pound rating is for a 24 hour period under 70 ambient temp with water temp at 55 degrees. Now if you have a water hose running on the ground and we all know how warm that water gets now when that water hits your ice machine you have already extended the freeze cycle which in turn will lower your production level.
When I had trailers long before I worked as a trainer for an Ice Machine Mfg. we always had 800 to 1000 pounds. Now if you have you ice machine in your trailer and the trailer is air conditioned you can go with a smaller unit. But if I had one in a truck I would go as large as I can.
Also if you find a machine that is less than 5 years old there is a good chance that the compressor is still under a parts warranty. School auctions, and new business closings are the best places to get a machine and I would always shy away from an ice machine that was in a bar. The reason for that is every bar has a customer or two that "are good with tools" and have already tried to fix the machine.
Pm me if you have any other questions.