You have power? I am going to assume here that you do and that you want a pump that will pump water through a garden hose to your garden location? Most pumps are rated two ways--rate of flow and the rise capability of the pump. So, like Zem mentioned, you are going to have to know the height you need to raise the water and the volume you want and size your pump accordingly. I like Little Giant submersibles. Even the smaller ones will give you a good flow rate if you do not have a big vertical rise. It is fairly easy to pump water when you are pumping level or downhill. It is when you start to raise the level of the water that you need the extra power.
I have no doubt that you can find something that will work just fine for well under $500. I would, however, be for putting it in a bucket that has holes drilled in it to allow the water to enter. This will go a long way to keeping any algae from being sucked up and interfering with the operation of the pump.
No reason that you cannot keep it stealth. You can run everything underground if you want.