Airport Express and poweline adapter

As you said using "powerline adapters" may be a solution. The problem is that some work, some don't, and there is no way to predict how it will work in any particular installation. Your first order of business is therefore to purchase adapters from a vendor with a reliable return policy.

Once installed, configure the Express to "connect using Ethernet" since you will be using its Ethernet port to connect to your router.

The following explains in greater detail.

Wi-Fi base stations: Extending the range of your wireless network by adding additional Wi-Fi base stations

Note Apple recommends the following in the above

If you have configured your Wi-Fi base stations in the past, it may be helpful to do a factory default reset of each Wi-Fi base station that will be part of the wireless extended network before you begin.

To answer your question, it is not strictly necessary, but it may be easier to start that way.

There have been significant improvements in power output from the fourth generation to the current version, the most dramatic of which are evident in the 5 GHz wide channel mode. My personal but completely unscientific test, which consists of walking outside holding a MacBook while counting steps until I cannot connect any more, appear to support that claim.

This means you should enjoy in better signal coverage in your particular installation but there are so many factors that affect this it is impossible to say for certain.