A long game would definitely be best, as the others have said.
On the part about standalones or sequels, you could always sort of do both for your story. For example, write the first part of the story, and see if you can tell the whole thing in one sitting. From the beginning, to the middle, the climax and the resolution of the problem or defeat of the villain.
Then, if you still want a second part, make it take place after some time has passed chronologically. Perhaps introduce a new problem or bad guy, so that way the readers don't have to have a huge exposition dump on the events of the first part to completely understand the second. A little bit of info is okay, but if there are pages and pages and huge paragraphs it would be an issue.
Extreme amounts of exposition and trying to cram large info dumps in readers brains would definitely make them close the browser and find something else.
This way it will be fun to read, and maybe the readers will be pleasantly surprised when they find out there is a part two, or that they accidentally read the second installment first and decide to read the first part second.
Any way, good luck on your story!