I do have a article on creating a balanced diet for your fish here. It talks about a few noteworthy pellet brands and how to go about feeding fresh greens. However it does need to be updated with information about gel foods! I highly recommend using a repashy brand gelfood. These are easy to make and do not use gelatin as a binder, so it is perfect for fish even with sensitive stomachs. I prefer to use Super Green as a staple and feeding frozen blood worms and some pellets as well. Gel foods are known as moist foods; this means they are more easily digested by the fish due to their high water content and this lessens the chance of encountering GI tract issues that may in turn affect the swim bladder.
I used to use a net over my pond, and still use a net over my raised smaller tub to prevent predation. Because my pond is above ground and roughly 2 1/2ft high no predator can fish out of it, so it isn’t covered anymore. I’m also in a heavily wooded area so we do not get heron or any other fishing birds. If you’re in an area where you feel predation could be a serious issue I recommend coated wire mesh over the pond to keep out unwanted visitors.
As far as buying fish that are “fairly cheap” your best bet is to begin perusing your local pet or fish stores. Many of my fish are from either a mom and pop or a chain store, even a cheap fish can be a great pet. If you want fish that are to breed standard for grooming, showing, or breeding then you’re going to have to pay 35+ dollars as most show quality fish cost more than that (depending on breed) plus the 40-70 dollar shipping costs.