You have to sustain the bankroll of your platform else you'll be looking for a lot of problems along the way. You will also have to make sure that your welcome bonuses are generous enough to attract a lot of new patrons and players onto your site. Your platform must also be seamless and the UI/UX must also be top-tier for you to attract a lot of new players in the game. There's a lot of things to consider, and with it comes a lot of $ too. You can always start with a couple thousand dollars and toss a coin up in the air for luck, the question is: how long can you manage to stay afloat without having some backlogs on your payouts?
And of course, building a website is not an issue, you only need experts for security and others, which is not a big deal. However, about the bonuses and payout, they are still not a big deal. You should know that bonuses are not real money, they are virtual, and even the conditions attached to them would make it difficult for anyone to win. Also, you should know that casinos could set their programs to 100% loss if they want, it's just an algorithm, which is different from sports betting.