1987 Centurion Lemans RS, only year that had the yellow and black, which tends to bring the best prices.

Original equipment was actually 2x7 Shimano Light Action, with Exage crankset, calipers, and hubs, laced to box section Araya bronzed rims.

Appears your bike has the OEM frame/fork/headset/stem/bars/saddle/post and wheelset.

The 2x8 105 stuff is an upgrade by about 4-5 years, the STI shifters and that 105 stuff is very nice, very reliable.
Now, if the wheelset is a 6-sp, it shouldn't work too well. However, if that's really a 7-sp rear freewheel, the 8-sp shifters will work fine with a "dummy shift."

The Tange 2 frame is just a tad heavier than the more aggressive Tange 1 frame on the Ironman. The Lemans RS is a great all-around bike, can be ridden far, fast, about anything you want to do. Handsome Mangalloy lugged fork, which I've always liked. Even the saddle is fairly comfortable.

I don't know the exact MSRP, but it was a sales leader due to the Ironman being relatively expensive. Around here, they went for $450-$495.

Right now, an STI-equipped bike, even a 2x7, should bring $250-$300. A Centurion, to some folks, will bring more if it's a Tange 1 Ironman or a Tange 2 Lemans RS.

I've sunk about $450 or more into one of those, and could probably sell it for $450 to people who know me, know it will be ready to ride.

The bike is fine right now, if that's a 7-sp rear freewheel. The possibilities are pretty amazing, as you can simply swap in a modern wheelset with an 8-sp cassette and drop some weight, gain a little range, if you so desire.

The bike will clean up well, and has a good quality coat of paint. There are a few folks here with the same make and model, all of whom like them. It's a good, solid road bike, is lighter than many other steel models that cost more, and this one, upgraded is nicely equipped.

Welcome to the forum.