There are many ways to poison a person effectively. The whole point of using a slow-acting poison like thallium is that it's hard to detect. Thallium is apparently uncommon enough that doctors didn't even bother testing for it until days after Wang (a prisoner who was poisoned with thallium) entered the hospital. It's a time-delay kill strategy. Arsenic is one of the classics. Hugely popular up to Victorian times, it was both easy to acquire and effective. It seems they couldn't get enough of it, being a common ingredient in any number of household products from wallpaper to paints. Arsenic poisoning occurred accidentally from all these and natural sources with notable cases such as the loopy King George III of Britain.
The superficial effects of arsenic poisoning are jaundice and a skin rash, but this tends to be after a long build up. Acute poisoning from a high dose results in intense gastric distress – basically everything goes wrong with your gut causing vomiting, diarrhea and bleeding, along with pain. Death follows convulsions and coma with circulatory failure being the ultimate cause. This may happen within a few hours of ingestion. Hemlock is always a good option. All parts of the plant are poisonous. Eating a salad made with hemlock leaves would be enough to cause death. Hemlock causes a gradual weakening of the muscles and intense pain as the muscles deteriorate and die. Though sight might be lost, the mind remains clear until death occurs. Symptoms begin in thirty minutes, though it takes several hours to die. Quail often eat hemlock seeds. They are immune to the poison, but the flesh from just one of these birds will paralyze a man. Why not have your killer prepare his guest a meal of quail that has recently fed on hemlock seeds? It would appear to be an accident.
There are many other options but these are a great place to start.