PCB assemblies (or PCBA as we call them in the industry) is not so bad. A very robust industry exists where companies can build and test custom PCBAs and cable harnesses at a range of quantities. Assuming you want your product to be durable, you will almost never want to assemble your custom PCBAs and cable harnesses in house. These days almost all PCBAs are surface-mount (SMT), not through-hole. Assembling an SMT PCBA requires placement robots and speciality equipment that will never make sense for you to acquire. You can use a variety of prototyping services to create hundreds of fully tested boards very cheaply.
Cable harnesses are a little less obvious. It would seem that having summer interns crimp wires into pins and then insert pins into connectors would be a cheap way to create your cable harnesses. There are two problems with this approach: First, you are looking at a lot of cable assembly equipment again, and the good equipment is not cheap. Second, you will have tremendous challenges with quality control if you make your own cables. It may look trivial to put a cable harness together, but you will be amazed how easy it is for even trained technicians to end up with quality problems that result in intermittently malfunctioning cables. For low quantities, find a prototype-quantity cable harness supplier to create your lots of 50 or 100 for you. For higher quantities, outsource to Asia. This is the one area where Asia is a viable choice even for moderately low quantities if you can tolerate the turnaround time.