You may have to do what gmmandan above did. Find a trailer in the size range you want, and if it has a 3500# axle under it, upgrade it to a 6000# axle. It can get to be tough to keep the weight down when you are getting rigged up and can't account for what everything is going to scale out at in the end. Not to mention if your total tooling weight exceeds what the trailer is rated for and you have to pick and choose what to take and what to leave behind when going out on a job. You will invariably run across a situation where you will be needing something you elected to leave behind to reduce weight. Then you may have to make another trip home to get said tool that you at first didn't think you were going to need. You may also need to carry material for the job on the trailer, thus reducing the amount of tooling can take with you. So I think it would be better to have a trailer rated for considerably more weight than just your basic needs for a job. That gives you more wiggle room when it comes time to add more tools to your set up in the future over what you think you need at the present time. A tandem axle trailer may be worth considering now, so you have more capacity built in from the start rather than having to upgrade in a year or two as your operations expand(hopefully in the future). So don't limit yourself to the 3500# weight rating right from the start without considering what your needs may be in the future. A heavy capacity trailer can carry a lighter load better than a light capacity trailer can carry a heavy load all the time. Just something to think about when trying to decide what trailer will fit your needs now and in the coming years.
Who knows, you may run across a good deal on a real good machine and have to pass on it because it would overload your light weight trailer that you didn't consider when trailer shopping.