Would welcome some input with current dilemma old floorboards

In middle of complete renovation on my 150 yr old cottage, 1st floor
is boarded with not very nice old planks 15mm, butted together, bit
wormy, quite bouncy, odd bit of rot, not too bad but not pretty. I
have extended the floor with new joists over the old stairwell.

Question: do I rip all old planks out (maybe rescue some), and refloor
entire floor with presumably floor-grade chip, 18mm or 22mm, and make
a proper job of it, or do I treat the existing boards and cover with a
thinner grade of chip - does floor-grade chip come in thinner
thicknesses? - and make up the brand new part of the floor with
standard 22mm floor-grade chip?

Opinions welcome, and I'm on a budget, so I have a feeling the latter
course of action might be better...

Cheers, Tom