Pretty clear what you want, but don't you think that for all these things your budget boundaries are a little tight?
I mean, the specification and functionality are pretty straightforward, but with all deployment platforms, testing, installers for all of these, there is so, so, so much work with all the details.
Only because of this it's hard to bid on a requirement like this, with your budget.
Regarding the C/C++ option, it's probably because of potential Mac version (but you can do this with C++ too :), or maybe you really do not want to have any dependencies (except for the serial device), but in that case it's going to be a challenge to create multiple build for all targets.
In any case - no matter if I win or not..., good luck!