The minimum graphics card requirement for AC: Unity is either a GTX 680 or an HD 7970, w/ 2 GB VRAM.
You might be able to get an R9 380 to do the job, but you're going to be over budget and may need finagling and lowered performance expectations, and may still find the rest of your system unable cope with the task of feeding the card adequately in this game.
Your best bet is to look in the used market for one of the cards listed in the minimum requirements, such as eBay or Craigslist.
Also, unless your PSU is of good quality, expect to need to upgrade it to handle the load requirements of the new GPU.