Your current floors are beautiful and classic. I would get the additional flooring to match. Have you lived with darker floors? We have floors similar to yours (31years old). They are very easy to keep looking great. Our DD has dark wood floors. ( 3 years old). They are awful to keep looking nice. Every crumb and speck of dust is magnified against the dark color. I would never have dark floors.

I echo grapefruit -- I have never been a fan of DARK wood floors (although I've never lived with them) becasue I would be afraid they would show every speck of dust (this is because my DARK wood furniture does!). I like flooring to match throughout the house -- with the exception of kitchen and baths -- whether it's wood, tile or carpet, so I would go with flooring that matches your current flooring.

IMO, any room with a door can have completely different colors of floor, walls, etc. Do not worry about your bedroom floors being different any more than you would worry about your bathroom floor being different. In fact, anything that distinguishes one area from another can allow different floors to coexist. See the photos below.

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Pick a mid /deep stain tone, sand ALL first floor flooring on site, old and new........leave the second floor or bedrooms on first as is. Just decide if in the long term......it will bug the daylights out of you. : )

Every house has its issues and only you know how much specific problems will bother you. If the newly installed wood is of the same species and grade, stain mismatch shouldn't be a problem. Different boards will take the stain differently and that will occur in both the new floor and the existing floor. A bit of color variation is part of the beauty of real hardwood floors. If you have the whole floor sanded, you can go with a slightly darker color but stick to a classic medium brown shade and it won't ever go out of style.

New engineered flooring isn't going to be as durable as your floor with a good quality finish. Flooring manufacturers tout their multiple coats of finish applied in a factory environment as something that can't be duplicated in an on-site job. True, but that isn't all positive - scratches, dings and dents happen on site and factory finish has to be replaced, not repaired. Rather than replacing the existing, consider spending a bit more money and doing a high end finish. You might want to investigate the newer UV cured coating systems that are marketed as comparable to prefinished products.

When my hubby was curious about off color stained wood, I asked him how much the slightly mismatched stain color in our current house bothered him. He hadn't even noticed it and it's been there close to ten years since I removed the carpet exposing the original finish hardwood.

What has been already posted is good advice. I just have one thing to add---hopefully this helps your decision.

"3) We're not sure if (assuming we can match the same grade wood) the
matched hardwood would stain the same as the existing hardwood. (Not
sure which this may favor)"

I can guarantee you will not be able to match the hardwood in your home with new wood of the same species. Unless you can get that flooring which was manufactured in the 60's.

Reason? Wood from the same tree can be different. Wood from the same species will be different, even when the trees were growing side by side. Wood from different decades will be different.

Example: The left side is oak plywood(bought at Home Depot) in 1991. The right side is oak plywood(from Home Depot) bought in 2017.

Thanks to all the comments so far. I really do appreciate it all. Some additional background regarding the floors and their "life" -

When we moved in 10 years ago, the entire house was carpeted. We pulled up all the carpet and had the floor refinished. The existing carpet was definitely on the floors for a long time. The hallway took the brunt of the damage as whoever installed the carpet went to town nailing down the tack strips. There are gashes and cut lines all over the edges of the hall. The finisher that did our floors said that they could sand down the hall more to get rid of the "scars" but that we would lose an additional layer of life from them. We opted to just leave it as is. So based on that (without measuring), there's at least one layer less of life in the floors.

As far as space goes for the areas we are considering -

Existing area is ~450sqft

Area to cover with new hardwood ~900sqft

If we get new engineered hardwood to replace everything, that adds up pretty quick. If we match and add, we still need to sand the existing area and finish everything.

Based on that info, does that change anyone's thoughts on whether we should replace everything with new engineered hardwood, or just try to match?