I can give you some valuable advice, based on a lot of experience in launching this sort of website -- not only dating/sugaring.

Customer acquisition (getting members) is the most expensive issue you will face. It is not the cost of building the site or the elegant design.

Make sure when you launch it, you limit it to one or two cities only. For example, Sydney and Melbourne. This is because there is no value in having 50 girls in Chicago and 50 men in Sydney and no real opportunity to connect with each other. Start small and focused.

The biggest challenge is getting members in large numbers very quickly.

The other issue is you will have to expect to offer the site for free for at least 12 months, so you need to have cash to live on while your membership grows.

As a startup, your competition will NOT be the need to compete with existing sites but to compete for attention of girls and men to join.

Then you have to think about why should any man join when seeking dot com already has thousands of girls. I think the main objection men have to seeking dot com is the cost where they charge around $120 USD per month and usually run promotions giving 25% discount. I guess if I were you, I would compete on price (say charge only $39 per month) because they are already killing you before you even start by having thousands of girls.

Good luck.