For espresso based coffee, do we need to stir, to mix the espresso and milk?

Are you asking as the barista (how to prepare)
or as a consumer (how to drink) ?

Then yes. This "mixing" is done at the initial milk pouring stage making sure the espresso and milk is well incoporated
If not, you may taste a bitter-ish first sip due to the crema of the coffee floating all the way to the top and not mixed with the milk foam

IMHO, whatever you like. You are making the brew for your own consumption (I believe) so brew it as you would enjoy it the most.

That being said, the common position is that lighter roasts do better pourover/filter/syphon possibly French Press/Aeropress. Espresso you will often see darker roasts and blends.

You should seriously consider single origins from smaller, independent local roasters; better quality for lower prices. Also support your local businesses.

Also, don't be too dependent on the roaster descriptions, unless you know they are reliable
Drink around and try same type of coffee from more roasters...maybe some other's you'll be able to pick it up

Try any natural processed Ethiopian you can find around. Start there