Ultrasonic cleaner:
I have a Cree 6 litre (1.5 US gallons) digital ultrasonic cleaner. I bought it based on several recommendations on this forum. It works great. It cleans both really well and quickly. My only real complaint about it is the heater. It takes for ever to get up to a useful temperature. To get around this I fill it with hot water. This works fine as I use it very infrequently. I suspect that all of the machines at this price level are the same in this respect.
Ultrasonic cleaning solutions:
Do not use any cleaning solution that is combustible. The ultrasound can cause them to produce explosive gases. Dedicated ultrasonic cleaning solutions are ridiculously expensive. I use Simple Green Aircraft cleaner. It is not the same as the regular Simple Green. It won't damage non-ferrous metals like aluminum, brass, bronze, etc., as the regular Simple Green can but it still cleans ferrous metals just as well. It has a very high dilution rate. That coupled with the fact that it remains effective over many uses the one gallon jug that I bought will last a long time. The bonus is, at least when I bought it, it was cheaper than the regular Simple Green.
Ultrasinic transdoucers:
Most of the machines designed for cleaning machinery parts, tools, etc., use 40Hz ultrasonic transducers. This frequency works great for machinery parts, carburetors, tools, etc.. For cleaning jewelery 60 Hz or higher is usually recommended, often 70 or 80Hz. 40 Hz is often cited as to aggressive and has the potential to damage the surfaces of some jewelry. Newer machines are available that are marketed as multipurpose ultrasonic cleaners. They also have 40Hz transducers but can be set to gentle mode which they claim reduces the power so that they can be used for more delicate items like jewelry. These were not around when I was researching so I don't know anything about that claim. They are not any more expensive than the fixed intensity ultrasonic cleaners so worth looking in to.
How's that for a start!