I won’t attempt to offer advice. I’ll just attempt to answer your questions.
#1
To create a map from premade assets, you will need only the time necessary to place the assets and lights and bake the lighting. For a small map and limited expectations, this could be done in as little as an hour. This estimate does not include any time that you will have to expend learning the editor functions or the process of importing assets.
As far as AAA, that depends entirely upon your own creative talent and your ability to follow Youtube tutorials. My estimate of the added time necessary to achieve that quality is in the range of hours to days of actual project work, according to your skill and talent. This estimate does not include tutorials or look up time.
#2
To swap the map out in a fps template game is simply a matter of changing the value in the Project Settings and perhaps dropping in a Player Start Object. You would then have the fps character (arms) running around in your map. Mind the edge. You will fall off unless you have arbitrarily blocked it. You would have access to all the basic functionality of the fps template.