I can't trust an invisible fence. Besides a dog ignoring the little jolt or getting stuck on the wrong side of the wire, there is the good possiblility that the wire will be damaged. One squirrel gnawing and the entire system is down.

Maybe you could put in a chain link fence in part of the yard, as much as you can afford right now, and expand the size as you can find the money for a larger size. If the neighborhood is growing into your area, the problem of other animals (or people) will grow also.

I fenced my back yard of about 40'X80' with treated solid wood for about $2000. Chain link for the same size would be far less. I did the work myself and it has held up for 12 years.