The best option i think would be choosing a company with both PCB and PCB assembly capabilities.
And for producing PCBs it is simple that you pass on your Gerber files, when you want additionally PCB assembly services, simply pass on your BOM list, with all the assembly components inside.
The start up quantity various from company to company, so you just have to discuss with them. And of course everybody is looking for a long term partnership.
Visit YuCha Electronics
View YuCha Electronics' Products