Are going through an interior wall, into the attic and out the soffit? How long will is the exhaust pipe? Are you going to seal it in the winter?

I have no experience with portable AC units, so I can't comment how well they work. My concern would be if the unit will provide enough cooling for all of your computer equipment.

Is it possible to install a window AC permanently in the wall on the side of the house?

The mini-split would be the best solution. I think it would run more efficiently than the portable device so over the long term you will save money. It can also provide heat if you need it in the winter.

I have an AC unit in my office which is 5 years old. Is it time to junk and buy a new one? Will I save on my energy bills. Cooling is ok, not too cold.

I was thinking of the same way vent through the wall into attic then out through soffit. The vent would only be about 4feet. I can't see why this won't work. I am not venting into attic just going though. This way keeps the big vent line hidden.