1) What equipment do I need?
Liner, water, fish...that's all you
need. Probably more ponds in the world are kept this way. But these people don't generally post in forums or write web pages. So reading forums and web pages can lead you to think most ponds have lots of equipment or need these things.
The real questions are things you may not yet know to ask. Like how do you feel about green water? Plants? How much maintenance? Stuff like that is where adding equipment can help...or hurt.
2) How deep/big of a pond do I need in order to have 5 - 10 fish and a few turtles
That's kind of an owner choice. 25-50 gals can be enough to keep 5-10 feeder size goldfish over summer and a turtle would probably be OK with that if it also had a proper out of pond area. Fish die in the the winter and are replaced in the spring. People do this, I don't want to get into judging people.
5-10 feeder goldfish can grow into large fish, like 10-12". A single 12" fish is about the same as say 30-50 feeder size goldfish. So the pond size kind of has more to do with your longer term goals.
3) What happens in the winter
To over winter fish the bigger (volume) and deeper the pond the less risk to the fish. Cleaning the pond in fall also reduces risk. It's not really possible to say size X for N fish is right. It's a risk thing. I'd bet 25-50 in NJ probably won't allow the turtle or fish to survive. I'd be thinking more in the 1000 gal range, maybe 500 gal depending on different things. Min of 3' deep but deeper reduces risk more. More volume means it can hold more O2 and dilute harmful gases more. Deeper water is warmer water since 39F water is heaviest.
Like I said there are options.
1. Replacing feeder goldfish in the spring is by far the easiest and cheapest. Many people would consider that harsh but once you understand 99% of Goldfish are killed to support our hobby it becomes harder to start throwing stones.
2. Bring the fish into the house for the winter, build a small tank in the garage and heat. Many people add pond heaters and things to keep holes in the ice but these things have big draw backs that are rarely considered. Things like water temp.
3. Learn a lot about the hobby and build a pond best you can to over winter the animals.
4) What start up cost would I be looking at (for things new)
You can get a new PVC liner for say $75. Feeder Goldfish are something like 10 for $1. Water. It's the optional stuff that starts to cost.
5) What monthly/annually maintenance costs am I looking at?
Water cost is pretty low. Electric depends on the toys and how long you run them.
6) What daily/weekly/monthly/annually maintenance activities am I looking at?
Depends on the pond size and how you want to keep it. For a small pond a simple minnow net (those green deals in the pet store) work great.
Is there a good to read getting started guide?
There are a lot of web pages and some books, but I personally consider them pretty poor. Most just repeat myths. The other issue is there are many kinds of ponds. The same feature can be perfect in one kind and the worst thing you can do in the other. Most web pages and all books look at all ponds as being the same kind. That makes it very confusing for people.
The best thing you can do is consider what kind of pond people are talking about when giving advice. Almost always they will be talking about their pond type.
To give you an idea on the differences. Here's a small pond I kept: 30 gal, 6' x 3', 10-12" deep, 25 watt pump (but ran with no pump for long periods. I kept mosquitofish in there, Goldfish would have been tough.
That was in San Jose CA. That size would freeze solid in NJ.
