There are different levels of assembly you could use.
Let's assume you get yourself a starter kit.
That consists of the Arduino, some single parts and a breadboard.
This breadboard is what you need to connect the different parts, using the wires which are also in the kit.
That way you will learn a bit about electronics too.
Some kits have small PCBs too.
The Arduino is built to allow PCB or modules to be plugged in directly, these are called shields.
If you're absolutely not interested in electronics, you can just use these shields, and use them a bit like lego builing blocks.
I guess a display shield or a display/keypad shield is the most spread one.
It allows you to play around with the display and try all kinds of stuff but it comes in very handy while debugging, as i already discovered (been playing with Arduino for less than a half year).
These shields are also inexpensive and you can get yourself a new shield when you think you are ready for a new step in learning so you can spread your expenses and keep your wallet safe.
Most of us do that, as it's fun and somewhat addictive to play around with this stuff.
Browse a bit in the shops you've been linked to and see what kind of shields and small sensor stuff is available (and at what prices).