How much of an extra cost premium is paid for assembly in the USA or UK?
If you do a 2 sided PCB and plan on doing one sided mounting all SMT except for minimal throughole.

Then, when you order your PCBs, you can order a solder paste template. You can put together a wooden holder with a bottom into which the board fits exactly. Pop in a PCB, place the solder template in there, then put some solder paste in there and use a squeegee (you can get them at a paint store) cut to fit it, and then just spread the paste over the board. Then remove the template and the PCB should have the correct paste on it (remember to clean the template). Then just place the components carefully.

For an oven, you can use the Black and Decker Infrawave series. It's important to get a good temperature profile (to not heat up too quickly or cool too quickly). There are many articles online on this oven. You can also use the Cuisinart convection toaster too.

So, you can do 4 board easily at a time this way, easily in 10 minutes if they are small boards. Thats 24 per hour. You hire someone at $24/hour. Thats $1 per board!!!!!!!
It'll take one person a little over a week to do 1000 boards. Recommended you use diagnostics in your flash to test the board and light an LED. You can always load diagnostics, run the test, then load your desired code and lock the MCU....it you are trying to save space. Testing the boards is the slow part perhaps.

For enclosure, you'll want to use an off the shelf plastic box. I don't know of a cheap fast way to do your own enclosures. Vacuum forming can work, but it's still slower than 24 per hour. Remember 24 per hour is $1 each, 12 per hour is $2 each, etc, etc...

Good Luck. Post if you find other info.