How deadly is poison in your campaign? it seems powerful at the extreme ends of herbs listed. they are rare and expensive i guess.
There are a lot of good posts in this thread with good ideas for addressing this. Which flavor of RM are you using? There Core books and the Companions for RM2 have all of the information that you are asking and we've always followed the stats given; potency, Addiction Factor, difficulty in finding, how it needs to be prepared (brew, powder, oil, ingested, etc.), cost, and so on.
The limiters you listed were the limiters we had for finding or obtaining the poison. The more extreme ones were extremely difficult to find or were very costly. If searching for them, the player needed Herb Lore, Region Lore, Flora/Fauna Lore, which ever was most appropriate, sometimes combinations of those skills.
Next the player needed the proper means to prepare the poison. Sometimes we used Cooking if it was a brew or a paste to make.
Finally, we have Use/Remove Poison skill. It's one thing to have the poison, but if you have anthrax powder and pop the top off your phial and accidentally inhale spores, then you really have anthrax! If you forget that you only have 20 minutes before the paste dries up and is useless, then you're stuck with a crust coated blade. It was also important to know how to clean up your tools and area once you were done with making the poison.
It seems like a lot to read out, but in roleplaying it, it's not that bad. What type of poison is the person looking for? Roll Herb Lore to see I the player pulls from memory a poison that has the effects he wants. Region Lore to see if he can locate the environment that will give him the best chances to find the ingredients and the GM checks to see what the difficulty in finding the herb is.
Prepping; roll Cooking or Use/Remove Poison, or both.
Deliver the poison by blade or brew.
Buying a pre-made poison from a guild is a different set of circumstances. As the GM, you can just decide if that guild, that member, has access to that particular poison to sell in the first place.