You might look into middleware options such as HeroEngine. License for engine with source is $75,000 according to their license page, but they also offer HeroCloud (cloud-based, access to tools but not source apparently) for $99 a year (plus 30% royalties on billing transactions), which would probably be a little more manageable for a startup. Of course, it's Windows-only. If you want something that's a little more developed (or so I understand), and more tested in real-world solutions, BigWorld might be more appropriate. It's the middleware behind World of Tanks. They offer an Indie license for $299 a year plus 10% royalties, or a full commercial license (have to put yourself under NDA to get pricing, but I doubt it's cheap). You're probably not going to find a ready-to-go full middleware engine that meets all of your exact requirements, however, so your best bet would be to go for one of the licenses that includes source for whatever middleware you use, in case you need to do source modifications to meet your requirements. (The BigWorld Indie+Source option is $2999 a year plus royalties.)

Dawoodoz was correct, however. This is not a small project you are taking on.