since you mentioned there is Cat5 cable upstairs, setting TL-WA901ND as an access point is a proper choice which can provide you wifi connection upstairs.
we may set same SSID with your original wifi, while it needs your wireless adapter supporting roaming function.
in that case, it can make you connect wifi automatically when you are walking around the house.

Simon can you elaborate this line on post 2
while it needs your wireless adapter supporting roaming function. to undestand the meaning and function.

winrow3745 wants to connect to internet automatically when he is walking around the house.
For example, he has two wireless devices and they have same SSID and password.
in that case, it will connect to wifi if the adapter supports roaming when he is moving from the wifi section of one device to other device's.
Hope i make it cleared.

on it will connect to wifi if the adapter supports roaming i have never seen a Tp-Link that supports roaming
only Unifi 3 line from Ubiquiti, the only one to do Zero Hand-Off Roaming technology

C/P from other source
the handover between multiple APs on same SSID often causes problems as the client will stick with the original AP even if the signal is well below the
"good enough" threshold. E.g. if you move your laptop from one end of the house to the other it will not switch simply to the new AP when it finds the AP with the much stronger signal, rather it will stick with the original AP until the signal is so weak and erratic (<5mb) that it can non longer be used. In many cases the signal may bet so weak that the laptop can not surf the net or communicate with network devises but laptop will still use original AP as it can still see a very weak signal. To fix this requires a manual intervention to force AP switch (eg repair or reset of wireless connection on laptop)