Please find answers to your questions herein:
How do market to a global user-base?
Well there are a variety of ways. In my 14 years experience in the online gambling / e-gaming field the most successful of operations use multiple methods such as; press releases, affiliates, sponsorships, social media, purchased ads just to name a few
Any ideas on how much capital is needed to get started?
This is a more complicated question. The expenses to keep in mind are; legal structure such as corporation, banking, cc processing, ewallet accounts, servers, software systems and most importantly marketing.
Are turnkey solutions better alternatives to in-house?
If you are referring to software I would say yes. The biggest benefit of obtaining software from a well established company is the amount of testing and real world use testing that the system has undergone. This will usually ferret our bugs, security holes and the like. So you get the benefit of this instead of having to undergo the time and risk yourself.
What are the infrastructure needs?
Again depends on a variety of factors.