My advice would be to buy one that's bigger and more powerful than you think you need. The first one I got for brass cleaning was a model a lot of reloaders seem to use and although it did the job OK, it took much longer than I had patience for and didn't have the capacity to clean more than 40 or so cases at a time.
So I then had a look around and found there are much better U/S cleaners around, with multiple transducers, drain taps, adjustable heaters, variable timer etc. and got one which has turned out to do a much better job of cleaning in a shorter time and could be used to clean much larger items if I need it to.
It's similar to this one:
http://www.ebay.co.uk/itm/Brand-New-3L- ... 27ce2ee46b
As for what to put in it, I use Citranox for brass, which is a highly concentrated citrus-based cleaner (eco-friendly). A large bottle lasts for ages but ti's not the easiest cleaner to get hold of. There are plenty of other ultrasonic cleaning solutions on the market, but ideally you should match the cleaner with the type of cleaning you're doing. Some are better at removing grease, others carbon, some shouldn't be used on certain metals etc. etc.
Hope that gives you a bit of a head start.
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