Some fencing questions

I live on a 3-acre fenced property, and want to fence off a smaller area within the property for my dogs. Since there's already a fence around the property and the dogs are never unattended outside, it won't be a disaster if they somehow get out of the enclosed area, so this new fence doesn't need to be super secure. The perimeter of the area is about 300 feet, and I'm thinking of using 4 foot high fencing. The fence will probably be removed in 2 or 3 years, so it only needs to last that long. I've never done something like this before, so I have a few questions:

1. What type of fencing should I use? The main options I've seen are chickenwire ($75/150ft), welded-wire ($70/100ft), and livestock ($140/100ft). (There's also cheaper plastic fencing, but I'm worried rabbits would chew it up.) I'm leaning towards welded-wire, but if I was sure chickenwire would be ok I could save some money using it. The livestock fencing seems sturdier than I need. Are there other options to consider?

2. I was thinking of using those green T-posts (like this) for posts. How often do they need to be placed? Like one post every 15 feet?

3. How do you attach the fence to the post? Some of the posts I looked at have little tabs that look like they should hold the crosswires of the fence, but the tabs and crosswires don't seem to line up (I looked at several different posts and fences at a store, and none matched). Other posts don't have tabs, but little nubs of some kind (like this), so don't seem capable of securing the fence. So are you supposed to use ties of some sort?

4. What's the best way to get the posts into the ground? Will I need to do any digging, or can I just hammer them in?