For head and shoulder shots, since you have room to back up, the 105 would be my choice and not the 50 since you need to be closer and it does not give the compression effect that a telephoto does that is flattering. These will probably be head and shoulders with shoulders not square to the camera axis. If you need to use fill flash with your built in flash, you will need to use the 50 so you can get closer.
I would mostly be concerned with hard shadows which produces a contrasty image that is often not flattering. Wide bright eyes will require shade and them being in it long enough to open the irises. For the individual shots you can use a white bed sheet or table cloth stretched over a frame made of 1/2 in od PVC pipe in a square using 90 degree elbows and set them on a stool to get them to a good height for you to shoot possibly sitting down. Position the shade so it shades the subject. If you are not familiar with manual shooting with a dark foreground and brighter background, use your built-in fill flash and Matrix metering so it exposes the brighter background correctly and the fill flash brings the face up to the background level. That way you can have shade adjusted eyes, no hard shadows and proper exposure in both the hills beyond and the subject. If you are using fill flash you will shoot at lower shutter speed so stopping down is required to get proper exposure. It sounds more complicated than it is to do. Once you set up, all will use the same settings. Go a day before and test it, use a hairstylist's wig dummy head, available at any beauty supply house for about $6 so you can experiment without boring a model Stan St Petersburg Russia