If you wish project to calculate the start date of a project, the first step is to ensure your project is scheduled from a finish date. Project tab > Project Information, Schedule from: Project Finish date, and set the finish date. Project will apply an As Late As Possible constraint to all tasks in the project and they will all initially be scheduled to finish on the finish date of the project.
The next step is to enter your tasks. Based upon your requirement, I would suggest fixed work task types. To set the default task type for all tasks go to File > Options, Schedule and select Fixed Work. Project will automatically set the tasks as effort driven, meaning if you add resources, the duration will drop and the amount of work on the task will stay stable.
Then link the tasks (predecessor/successor) to create the logical relationships between the tasks. I wouldn't worry about duration yet as we'll let Project calculate the duration by specifying the work and resources. Create the resources in the Resource Sheet view, setting the Max Units as needed. If you have a pool of 21 welders, set the Max. Units to 2100%.
If you know how many work hours and which resources are required, split your screen (View tab, select "Details".) and select the resource's name -- enter assignment units (if it is other than 100%) and specify the work. Click OK and Project will calculate the duration. Because you are scheduling "backwards" -- Project will also calculate the start date.
To the due dates on tasks -- setting the predecessors will start to drive dates. You can set constraints, but particularly in early planning -- I would be more inclined to use deadlines.
I hope this helps.