If I am not mistaken - the ACE 150 does not have the pinpointer mode that the ACE250 has.
It is worth every penny.
I got a MD for Christmas - never told my hubby what I wanted (I had no idea anyway) and he got me the Discovery 1100. I think it was around $100. After 1 month I knew I needed a better machine because I loved the hobby. I bought the ACE250 in Feb which is a fantastic machine for the price. So basically, if you KNOW you love the hobby then try to save a little more and go for the new ACE250. If you aren't sure, then you might want to try to borrow one from someone and see how you like it first.
Not in my experience. The old Ace series is 10 year old technology, clunkier, heavier, and not as pretty, and most importantly not near as deep. I had the Ace 300 and had lots of issues with it.
Nothing new for that price. A used White's Classic III would be a good bet. It has much of the depth capability of the XLT, MXT, and DFX without all the bells and whistles(programs and ID technology) Look on eBay. It will pay for itself in no time plus enough to invest in an upgrade.
I bought a new Ace 250 on Ebay for less than $200...
dollar for dollar I'd say save a few more dollars and get that. Better than the Ace 150 and you will definately like it. You also might be able to get a used 250 for your price range. Good luck and hope you enjoy your detector, no matter what it is!!