What CAD system did you use before? What were you doing in automotive design? Will help us understand where you are coming from for past experience and frame our answers accordingly.
Assuming since your current employer knows you have no SolidWorks experience that they are planning on sending you to a SolidWorks training class soon?
In the meantime, I would recommend that you go through all the SolidWorks Tutorials found under Help in SolidWorks.
Also check out www.solidprofessor.com for some good video based training.
I am agreed with the answer that create the base part and call it in to the assemblies and by taking the reference create the new part in to the assemblies. Top down assembly is simple method to avoid the confusion.
While using the weldments try to use the 3D sketch command. It will help you to create the multiple profile
And Chang the profile orientation by using the locate profile option then you will get proper weldment orientation
Get ready for a big re-learning curve. I have been in the same boat as you. I'm a long time AutoCAD guy and the entire process is different in SW. Not really hard, it just takes a bit to get your head around the process. In AutoCAD everything was (often) in the same drawing where you could quickly see the relationships from one part to the next. In SW (for the most part) you will have separate files for each part.
You will have to fully create the 3-dimensional parts. Then combine them in an assembly (or a sub-assemblies) where you mate them together. Then finally you will have another file(s) for the actual drawings. In the drawing file(s) you will insert the assembly, sub-assemblies, or part to be dimensioned and detailed.
I would strongly recommend doing the tutorials and if you can, get into a class. I prefer actual classes given at schools (collages, tech. schools or community colleges) rather than VAR's (Value Added Resellers = Software distributors) just because you have time to digest what you went over and let it sink in. Plus some of it is beat into your head doing it several times. And there's the benefit of getting graded feed back on what you did right and wrong. Some VAR's might provide that part but i have never seen it.
A couple things that gave me trouble at first (before I understood their importance) is fully defining as much as possible. Try to fully define sketches as you make up parts. Try to fully define assemblies & sub-assemblies when mating parts together. When mating try to think about what might change as your design evolves and try to mate to global constants if possible. What I mean by global constants is stuff like your X,Y, Z planes , your origin, or sketches created in the assembly just for the purpose of mating. If you mate to an edge or corner and the design changes to where that edge or corner goes away your mate is broken. Sounds simple to replace but often you can mate things to things that are dependant to thing you already have mated. When that happens it's a nightmare to try and figure which thing(s) need to be fixed.
There are a lot of advantages to both AutoCAD and SW, they are just different and take a little time to get understand how and why they act the way they do. The more you learn the less problems you will have with SW.
For some tasks, using a multi-body part as a master model and then splitting the bodies out into parts for final details and drawings works well.