Social enterprise and "most profitable" are in conflict with each other. It may be best if you focused on one or the other and once the idea works, then either make it profitable or add a social component. Trying both at the same time has a very high chance for failure.

Depends on your industry and target customer. As a no-revenue start-up I would not spend more than $ 1,000. If it is a consumer direct item then I would put most of my marketing money into ppc ads and still less than $ 1,000 for SEO.

That will depend on qualiy, price and availability.

The issues are:
1) you have to sign up thousands of suppliers to cover geographic issues
2) You need thousands of event planners to make the platform be of interest to suppliers
3) How will you monitize this (i.e. membership dues, commissin on each transaction)?
4) Tech stack needed is expensive

There have been many stories of successful examples in the news. The big one is the "Loess Plateau" in China, the "Altiplano" in Bolivia and the "Green Belt Movement in Kenya.

First you need to calculate your costs.
Then estimate the number of transactions per month
Then estimate your cost and margin per transaction
Identify who benefits the most and charge them 66% of the number you need and charge 34% to the other party.
Monitor and test and be willing to change percentages based on what works.

I have taken a manufacturing company from initial capital raise to exit sale. It took 137 pitches to raise low 7 figures.

First you need a MVP and a business plan. With that in place you can solicit investors. You start with your most inner circle (family) and work yourself outward from there. If you do not want to offer equity, then offer notes. In any case if you do not have a product or business plan, you are not serious.

Go to the source - i.e. monitor government sites that list new company incorporations and then call the companies with your pitch. In the US that would be the Secretary of State website for each state.

This is not the way to do it. You need a track record.

Since you are brand-new at this, ignore the AI generated answers. Because you have no base line, based on my actual experience from more than 10 years ago, my suggestion is:
1) Study other peoples profiles and what they charge.
2) Trust your instincts and set a rate that you think will work for you.
3) Check your statistics for a week, if no business or click on your profile,raise or lower your rate.
4) Repeat until your rate generates clicks. Then fine-tune your profile and do some outbound prospecting.