2002 Pontiac Montana Troubleshoot cause and how easy to fix

I was driving about 30 miles, and my engine temp spike quickly up to the red, and the car got sluggish, I pulled over, and when I put the car in park, the temp dipped about 20 degrees. The coolant recovery tank, which on my car is usually somewhat empty, was full.

I got the car home, but the next morning it worked fine on the same drive (about 40 miles each way). I know I shouldn't have driven it, but had no alternative.

It drove fine for a week with no problems and the temp right where it usually is.

Then today, it happened again.

Also, on both occasions I notice the fan does not blow heat.

I picked up a thermostat, but have not put it in yet. I heard that on a Pontiac Montana, it is very hard to change yourself, as you have to take off an exhaust crossover pipe and then the bolts are hard to get to. Is it that hard to do myself? I have a basic tools set but not extra things like offset wrenches, etc.

If I only have a short time late at night to work on this, is it better to take to a mechanic? If so, how long does it take to repair and a projected cost?