Hey....welcome to the site. You have come to the right place to talk about Ford engines, etc.
You'll have to do some measuring to see if the 5.0 will fit under the hood, and you will need to make sure the suspension on that Mercedes is strong enough to support the 5.0. Yes, you will need a matching transmission, as the Mercedes won't bolt up to that 5.0. You will most likely have to get another drive shaft from the donor, and most likely the rear end as well. You will need to carefully measure the rear end to make sure the Ford rear will fit. Or, better still, rework the donor driveshaft where it hooks up the the Mercedes rear, then you wont have to bother with changing out the rear, assuming the Mercedes rear is strong enough to handle the HP and torque of that 5.0. It sounds like an expensive project to me, but anything is possible if you can use a cutting torch and welder.