If you are mainly going to hunt on grass, the minelab vanquish series is a great bang for the buck. They stink in iron and broken ground though as they only run multi freq. I just sold one to a buddy and he likes it. I use my deus for everything nowadays as it is crazy lightweight. It's a single freq. but i can change the freq. to suit me for what i'm hunting for. Findmall is a good sight to learn on the different detectors as they have forums for each brand. Don't fall for the kellyco "package" deals as the stuff they include is mostly junk anyways.
In parks you are better off with a hand digger as walking around with a shovel will cause problems with somebody. Fill your holes nice and you won't lose any places.