I use the Sigma 17-50mm and their 50-150mm f/2.8 on my DX bodies. I have no problem.
I find Amethta advise is always on target
Since buying my D800,I use the Nikon 85mm f/1.8 most of the time.
I shoot mostly portraits. On the streets or studio set up.
I find this a great combination
Also I would pose the people
1st 6 people on the ground
2nd 6 people kneeing
3rd 6 people sitting
4th 6 people standing
5th 6 people standing on a step or bench
you can add to these rows as needed
keep the rows straight for greater dof
I would not use a C shape. The plane of focus is great if you have straight lines.
I would use the 18-200mm lens. Try to shoot at f/8 or f/11.
pose the people in rows and in the shade. Focus about 1/3 in. The dof should have everyone in focus.
Take a lot6 of images people move and close their eyes.
I use aperture priority most of the time. When not I use manual
Good luck and happy shooting. Great choice staying with Nikon products
join a good camera club in your area. You'll get all the help you want
I buy mostly from Adarama. They stand by their products. If ever a problem with the item (they take it back). No questions
If I had to take only one lens it would be Nikon 18-200mm
I would buy a Nikon 50mm AF f/1.8 lens. The cost is about $125, for a fast lens that is cheap.
I would use the lens as a zoom (the old fashion way feet}
Set to high ISO to increase the shutter speed.
After setting ISO look for a shutter speed of 1/250-1/500
After a few shots check.
Remove lens hood and filter. Then check it by taking the exposure at all f/stop.
See if vignetting is gone.
When flying, I use my Lowepro roller. It will fit the equipment you mention and more.
Don't forget card reader and portable hard drive
Great city. Don't miss. Great photo op. One of the best cities in Europe and one of the few I would revisit.