Garrett makes a great product. I think the upgrade information about the ACE Series is a phone call away. Call Garrett. I know that there are coils available for the ACE 350 and the AT PRO. I have owned and used both machines.
Instead of investing in upgrades on the 250, you might want to upgrade to the PRO. It will get you depths of 10 inches and the 5 X 8 coil can help isolate trash. There are some good used units that become available from time to time.