Personally, I think you need to determine what you ultimately want out of it before doing any replacement. If it is to be a rider, clean it up, lube it and go. The components on there are rider quality. Exage is lesser to 105 which is lesser to 600. But I doubt you'd see significant difference in daily riding so long as everything is adjusted properly.
If you intend to flip, then you have options. Option
#1 would be to put the original equipment on it. Go to Vintage Trek website and find the catalog. Option
#2 is to polish everything and sell as-is. Making it correct as far as original kit is not going to enhance the price that much, IMHO. I suppose a third option is as stated above, to at least get all components to match. I think that is also a losing proposition unless you score some cheap replacements or find a donor bike at a bargain price.
So decide what you want to do. Don't waste time and money upgrading if you can't increase the sale price enough to make it worthwhile. If it is a keeper, leave it as-is or upgrade to your heart's content.