I have heard about some ingredients which contain small quantities of toxic substances and that in excess could cause health damage, (for example I heard that sunflower seeds contains selenium and in excess selenium toxicity can result in symptoms of selenosis). Are there other vegan foods which could be dangerous in excess?
Yes, some foods (even plant-based foods) can be harmful when consumed in excess. Consuming too much of one food can displace other foods from your diet, making your diet less balanced overall. But I believe your question is about toxicity from individual foods assuming other nutritional needs have been met, so I'll provide a couple examples of foods that may be harmful within an average daily calorie budget.
Acute selenium toxicity can arise from eating foods that are very high in selenium. For example, a diet of exclusively brazil nuts (60 nuts per day) would provide a toxic amount of selenium that would likely induce nausea and vomiting.
Facial flushing (redness of the skin) could be introduced by consuming too much nutritional yeast, as described in the question Are there risks from eating too much nutritional yeast?
The tolerable upper limit for copper intake is set at 10 mg, and this limit would be exceeded if one ate 4 whole bunches of kale every day. If this was kept up over a long period of time, it could cause chronic copper toxicity. Of course nobody is eating 4 bunches of kale per day.
Many processed foods are high in sodium which can lead to high blood pressure.
In practice there is no need to worry about any of this as long as you're eating a varied diet that doesn't put too much emphasis on any one food. This is especially true when eating whole foods (as opposed to packaged and refined products) because it is difficult to consume whole foods to excess.
Soy in Excess
Often soy based products are used to substitute as many dairy alternatives, like soy milk, and meat alternatives, like tofu products. However, many nutritionists have been cautioning against the regular consumption of soy based products, as soy is naturally high in phytoestrogens, which is the organic compound that mimics estrogen. Thus, eating such foods have potential to not only create hormonal imbalances within an individual, but animal studies suggest that consuming large amounts of estrogenic compounds might reduce fertility in women, trigger premature puberty and disrupt development of fetuses and children. Other animal studies have seen direct changes in testosterone levels after soy was consumed.
How about plain old sugar and salt? Probably the most common dietary advice is to cut down on these.
Exxess potassium is lethal but I have never heard of anyone dying from a natural source. Banana are high in potassium but death by banana is rare if not unknown.