Congratulations on a great rifle. With respect to your first question, I highly recommend using authentic hk 300 mags. They are expensive; you can find them on gunbroker. Usually in the $150-200 range. I have read that the promags have problems feeding. I do not use any of these, only authentic HK mags. With respect to the buffer, there is a company that makes them. There are two styles for the HK300. Since you do not have a buffer to go by, look at your serial number. Make sure you tell them what it is as there were some changes with the HK 300 over the years. The early rifles are 15,000 and below, while newer are 15000 and above. You can also tell by the stock of rifles. Here is a link to the company that makes the buffers: https://www.critrcall.com/h-k-buffer-plates.html. With respect to the extraction issue, extractors I believe they may still be available. I am at work and can not remember the hook on the extractor. Check Midwest parts or Numrich on line. I have never had any issues with mine extracting. I use CCI, but can not remember the type of CCI ammo. I can tell you later today or tomorrow what I have used, but have not had any problems and did not use the ones you have described. I have not read if they are finicky with ammo and like I stated, the type of CCI 22wmr I use have problems. Please post pics also.