Kevin,
Don't let the threat of cavitation drive you away from an older machine. There are dozens of old Fords that have been in my local area, and I know of only a couple that have had the problem. Every brand/model of machine will have some weakness. The "Triple Nickel" Fords were known to be one of the best diggers in their day, and are much simpler than other brands. You could probably repair a cavitated block on a Ford for what you could spend on the hydraulic system on other machines. Keep looking for that 555 - you will find the right one eventually.
I won't quit on the current prospect. I need to learn a bit more about it and make a fitting offer. There are a couple of other 555x types available in the area, too.
Thanks for tips.
Kevin