Just as the title says, what kind of snake is Nagini? Note: Nagini is not the boa constrictor from Sorcerer's Stone / Philosopher's Stone. Boa constrictors are not venomous.
Okay. I believe I have deduced what species Nagini is, if she is indeed a normal snake, and not some obscure type of magical creature.
From the HP Wiki (I realize the Wiki is not reliable on matters of canon, but I have verified this separately): (Hindi: nāginī, pronounced [ˈnɑːɡɪniː]; variant of Hindi: nāgin, a female cobra, from Hindi/Sanskrit: nāg, a cobra). Nagini's name is almost a direct conjugation of the Hindi word for female cobra.
Her description in the book does NOT match her depiction in the movies. According to Harry Potter: A Pop-Up Book, the filmmakers chose to depict her as a "burmese python." (I assume the pop-up book is not pulling that out of its backside.)
Nagini is described in the books as female, green skinned, hooded, extremely venomous via some type of neurotoxin, and roughly twelve feet long, and thick as a man's thigh. This fits the description of an olive-green king cobra, in every aspect but the length and thickness. Large king cobras are capable of exceeding six feet in length, but this is uncommon. The Engorgio charm would be capable of increasing an object to roughly twice its normal size, based upon its seen usage.
I believe Nagini is a permanently Engorgio'd king cobra, or in her case, queen cobra. :)
Albania has very few poisonous snakes. Few that have anti-coagulation properties (which is the affliction given to Arthur when Nagini bites him). One that fits the bill is Vipera ammodytes, whose venom
Again on Wikipedia, its venom is described as having anticoagulant effects. In fact it's noted to be likely the most dangerous snake to be found in Europe.
I initially disregarded the snake as it was far too small to be Nagini, but if such a snake was the subject of Dark Magic who's to say it couldn't be permanently enlarged, giving it the capability and the sapience to constrict? That would be a dangerous snake.