Most of the hobby grade units ($120 -$300) are very similar in performance, with the upper end of that range having more options to discriminate targets. Don't expect to detect small objects much deeper than 16" or so. Most models are designed specifically to locate coins and small jewellery items, while a few are more oriented to detect gold nuggets.
I've had a basic Whites model for over 30 years, and still use it occasionally, though kneeling down to retrieve items is getting hard on my 75 year old body! I used to be able to do that for hours on end, but now an hour or so is about all I can handle. My daughter has a recent model Bounty Hunter Tracker, which performs about the same as my ancient Whites, and we get out together a few times each year to search older local parks and lake beaches, with reasonable success.
I've found coins all the way back to the 1890s, and a few nice rings and gold chains, but nothing worth more than about $100 in value. It is a fun hobby, helping to keep you in shape, out in the fresh air, and interested in local history.