Not a department store bike. Mt Shasta was, however, GT lower end line. I bought it as old stock from a LBS in Des Moines, Iowa way back in 1991.

Mine has a blackburn rack in the back. It has Shimano shifter (thumb and index finger style.) Does anyone know how I can post a picture?

The frame is probably fine though the chainstay might be a little short for heel clearance with panniers. There are ways to work around this if it's an issue. The components could be upgraded with time, but they should be fine also.

The only change you may wish to make soon is the handlebars, but that's really your preference too.

I prefer drop bars others like trekking bars. Since your bike has thumbshifters, trekking bars would make sense because you could use your current shifters and brake levers. Since you've already been using your bike for long rides, you might be fine without changing your bars. When I've tried to tour on my mountain bike, I found the lack of hand positions made my arms and hands tired over time. That may have more to do with bike fit than the lack of hand positions.