Electronic – Manufacturing, qualification and sale

So from the previously linked answer, I think the most interesting/beneficial things for me would be the bullet about learning the tools (compiler and linker), and learning different styles of software architecture (going from interrupt based control loops to schedulers and RTOSes)

Porting a small operating system to a new device could help you to understand schedulers and RTOSs. FreeRTOS is popular and well documented. eCos is another.

Writing a bootloader is a good way to get to grips with a linker as you'll want to divide up memory and flash into regions.

Another tip is to pick a completely new architecture or chip and build yourself a development board. Forcing yourself to start right from the beginning and look everything up in the datasheet is a good way to learn.

Explore Protothreads. Try writing the same programs in both a threaded and state machine style. Once you're done with Protothreads, write a real thread scheduler.

1 . Breadboard or Stripboard will be fine for prototyping/learning purposes. FOr the latter you need to have basic skills with a soldering iron.

1. Either should be fine for basic distance sensing. You have more control with separate units , for instance if you want the receiver and transmitter to have different polar patterns (sensitivity angle - you could put one in a tube to make it more directional for example) or slightly different angles (for whatever reason)

2. Can be powered by any reasonably clean ~5-12VDC >250mA supply (probably get away with less but 250mA is easy and makes sure you have plenty for additional stuff like LEDs, LCD, etc) This could be a DC wall brick, batteries, etc. You will need a regulator (e.g. 3.3V LDO) for your micro but the transducers can be powered directly (up to 20V rms according to Velleman page)
   You will need a couple of general purpose transistors (NPN or N-ch MOSFET - BC337-40 is a good choice) to switch the transmitter with, as you will want to use the higher voltage input rail (rather than the regulated micro rail) for extra range.

3. Any small 8-bit micro with a couple of timers, a PWM peripheral (a comparator/ADC would be nice) will do for simple sensing and display on e.g. LCD. This could be a PIC16F, PIC18F, AVR of some sort, MSP430, etc. You can get simple dev boards or "starter packs" for all these micros that will get you up and running quickly. For example with a PIC16F you can use MPLAB/MPLABX to write your code on, and use a PICkit3 to program the chip with. Here is an 18F pack that includes dev board and PICkit3. Lots more boards here.

4. I assume you mean using Proteus for development (I have only seen it's PCB design part) This is okay if you want to go that way, Proteus has a pretty good rep from what I have heard. Personally though, I would stick with the tools provided by the people who make the chips (unless they don't provide anything) I have used MPLAB for all my PIC development and it's a very solid platform. MPLABX is the next generation IDE, based on Netbeans.