We are in the process of filling our gaps with pine slithers (they cam in different widths) You can buy them online. You do suffer from bald/ red knees when you hammer them in and then need to chisel them down so they are flush with the rest of the floor but it really does help. What you can also order with the pine slithers is pine saw dust with resin - you mix the two up and can fill the really narrow gaps which the slithers don't fit into. If you don't mind spending the time doing this I would recommend it yourself (especially if you're worried about costs), depending how big the surface is, and then have someone in to do the sanding. The varnishing you can also do you yourself with a normal paint roller. We also recently had a quote for sanding and varnishing and I think it came to £500 for mid size living room and 3 m length hall.

Is the house on a slab or is it raised? Wood throughout is always a gorgeous pop of color since you are leaning toward a light wall color. Here are a couple of examples of light walls with various collections we offer. The flooring comes unfinished so it is a great canvas if you wish to create a custom look!

I disagree. This is standard whitewood T&G flooring. It's generally laid across joists by builders on the first floor of two storey houses as a cheap subfloor ready to take a floor finish. I've seen it sanded and varnished / stained and while it looks OK for a few months, it doesn't wear well as it's a (very) soft wood and is very noisy, particularly in the rooms under. If you want a wood finish without breaking the bank, there are some good laminates that you can lay over the T&G boards. A semi-engineered board would be better but will cost a bit more.

If I understand correctly you are adding extra floor by bay window feature ? Have you thought as out a window seat to hide the floor issue, can give you more storage and a really useful seating feature (great with kids)