New bass strings would probably improve the tone considerably, but that would be an additional cost.
I suggest that you keep looking to find a more suitable piano. Challen is an excellent make and other British brands worth considering are: Welmar, Marshall and Rose, John Broadwood and Sons, Knight, Collard and Collard, to name just a few.
When you find a piano which appeals to you, have it checked by an independent tuner/technician.
All the best for your search.