TL;DR: I have most of what you need already available or mostly done. I can work on the project before you even accept this bid. No strings attached, you can pay after that, this way you can see how I work without taking any risk. Just let me know and let's talk specifics.
Good day, I have plenty of experience working with MCUs (mainly freescale and STs families), especially with arm-based ones and lately with the esp8265 and esp32 family of microcontrollers. I've worked at Freescale Semiconductors for half a decade as an engineer.
I'd need more information regarding the exact requirements of your project. However, for what I understand at the moment, I can say the time is more than enough for finishing it and having a long enough testing phase. I have already all the required hardware available, because of previous projects. Also, I already have a working bacnet stack for MCUs, tested and functional on STM32 MCUs. I will not use arduino, instead I will use the standard library for the STM32 family, this will make the firmware much more robust, as well as make a better use of the hardware capabilities. Making the modifications required to fit your requirements will be relatively easy.
Of course, all payment can be after the delivery of the firmware. Also, I can work on the project before you even accept this bid. No strings attached, you can pay after that, this way you can see how I work without taking any risk. Just let me know and let's talk specifics.