I'm with a PCB factory based in China. I would strongly recommend you not to set up PCB manufacturing and assembly line whether if you're after small Qty or large volume circuit boards production.
You guys are specialized in electronics design and engineering, I guess you have no idea on circuit boards equipments cost, equipments maintain fee, material fee, production cost (including quality control, disposal and environment effects), human power expense, factory space expense, etc. The total cost is heavy and will be way out of your expectation.
You mentioned to take outside PCB orders when your lines are free. Well, this is possible for the PCB manufacturing line, but a BIG NO for the Assembly line. If you cannot perfectly combine circuit assembly orders end to end in the Assembly line, you have to restart the whole assembly line each time new order comes. The production cost will be super high (Even higher than your profit from outside orders. And your in-house PCB Assembly cost will be much much higher then order assembly service directly from the professional PCB House), your earnings and spending are very likely to be out of whack. Not to meniton the fiercely competitive PCB production markets, with som many experienced PCB Manufacturers & Assemblers of successful working records and increasing number of new PCB factories, it's extremely difficult for you to get business especially when you do not have cost benifits.
As for your case, I would recommend you find a long-term custom PCB production Partner. You can do some research on PCB manufacturers based on your project's requirements, budget, schedule, etc. and place a test order (small Qty of circuit boards) to see whether if this manufacturer can perfectly meet your expectation. Once you're fully aware of its manufacturing capabilities, reliabilities, you can rest assurred to submit your future pcb orders to them.
In case you changed your mind and want to find PCB production partner, we're a China-based custom PCB manufatcuring and assembly services provider for worldwide companies of all sizes with 10+ years' experience. We're committed on the quality & performance of each board we make. Feel free to check and get free quotation.