As Virtual1 says WDS can be pretty fickle, but unless you have a specific need to maintain the MAC (
Media Access Control) address of the devices on the network it is unnecessary. You can easily extend a Fi-Fi network without it.
I have a couple of distributed networks using a Time Capsule and multiple Airport Express that has been very reliable and functional. Instead of setting up a WDS my Time Capsule creates the network while the AEs rather than being configured to
participate in a WDS network are configured to
Extend a wireless network. I also have another AE configured to
Join a wireless network that I use to enable a non-network USB printer to work on the network.
This has been a very reliable configuration for me in multiple locations. In fact I can roam throughout the network with my iPad, iPhone, or MacBook Air without ever being aware the connection has been taken over by another base station. In one case I had some spare somewhat elderly Airport Extreme Base stations that I used in place of Airport Express and got even better range in the network.
POST SCRIPT: I just saw your last post and I would be more inclined to spend about the same money on two Airport Express and put one on the ground floor and the other in the basement. The problem in the basement is the two levels of floor the signal goes through will seriously attenuate the signal. Theoretically going to 802.11n and the 5GHz band should give you better signal penetration through the floors, but there are no guarantees. At my son's three story townhouse with concrete floors it took a station on each floor to get a usable signal to the third floor.