I have and Ace 250, and it is a good machine for what it is. The Ace is the best of the beginner's models and if the find is in the top 6 inches or so you should be able to find it. Unfortunately most of the old coins have sunk lower than that, so you will likely to find mostly clad and modern jewelry (which isn't bad at all!). I'm thinking of upgrading to a Minelab Explorer or E-Trac later this year.
The other limitation is if you take it to a saltwater beach. It will give you fits there.
That being said, if you are out detecting regularly you will make enough finds to pay for your detector in no time!