"Are there any online businesses that are completely passive that someone like me can buy (ie. Flippa) and allow to generate money in the backround with no work on my end?"
No, not really unless you consider some get rich schemes and churn and burn schemes. But even they require some work on your part. such as.....
Get rich quick sites,,
You're selling the idea of how to create a website that makes money. Personally i consider this a pipe dream effort. You don't get repeat sales ..... But people will sometimes buy anything.
( I don't recommend you attempt this )
Churn and burn sites..
You sell a product, typically a health food, maybe a fad diet scheme etc, etc. as long as it's something that is popular with the masses on the internet. You use pbn's to rank the site then make a profit for a few months then the site gets blacklisted. Then you rinse and repeat the process. People make huge livings like this.
This also requires work on your end.
"My thought is purchase a website with opportunity to value add, then hire someone to run it and grow the website with time. Is that realistic?
If it is possible, how does one go about that process and what type of business model should I invest in?"
Then there is the long haul as you say, Something with opportunity to value add.
My suggestion would be to buy a business on Flippa then hire people to run the back end of the business. Now you have a residual income and fixed expenses. You can then learn what you don't know but need to learn, then you rinse and repeat this as often as you like.
But do understand this .. if you're not managing some part of the business then you're managing the people that manage the day to day operations.
Either way you will be putting in work.