1. Similar things exist - like Houzz. But that doesn't mean there isn't a need - see point 5.
2. Probably a progressive web app (this is an app that's actually a website but looks like an app). It really depends on the key functions that you need.
3 + 4: I would not buy products and then sell them. Too much of a logistics and liability issue - unless it's drop shipping. Revenue streams: affiliate programs (for selling products), eCommerce (sale of specific products that are easy to store and ship), advertising, selling leads...lots...
5. See my answer on how to do market validation here: https://clarity.fm/questions/6423/how-do-you-do-market-research
I've successfully helped over 300 entrepreneurs, startups and businesses, and I would be happy to help you. After scheduling a call, please send me some background information so that I can prepare in advance - thus giving you maximum value for your money. Take a look at the great reviews I’ve received: https://clarity.fm/assafben-david