I do have good folks for plumbing, flooring, landscape, fencing and paint.

Make sure you have a good home warranty insurance policy (for plumbing, electric, etc.). 

The walls came assembled with the plumbing in them from the factory, and then they craned them into place.

You can see this all the time on trucks: 'ABC Plumbing - Specializing in remodels, baths, kitchens, tile, etc'What this means is that they are really not good at anything and desperate to take on any job just to make a few bucks.

Depends on the scope of the project, but I'd guess yes if a more extensive project or your messing with plumbing, electrical, or HVAC.

If there is already plumbing in the basement then convert that into another bath and kitchenette. 

Also to make it legal, you need to make sure you have sufficient egress, fire separation between upstairs and downstairs and technically you need entirely separate mechanical/plumbing/electrical systems.

Is there actual plumbing changes or are you just changing the physical sink? 

Would you pay to have the plumbing capped off at the source or leave it for the tenant to use?

It's worth around $450k & I'm not sure how long it'll take to sell in the current market.Our immediate family (my wife, her dad, out daughter & I) are very handy and have experience in construction, HVAC, electrical, plumbing, etc, so we are anticipating being the general contractor and subbing out only those things we cannot do ourselves.We have several STRs, but no motels.