Yes Cosmo is the best location, most popping. You're right next to the Bellagio, City Center, the big ass mall across the street. I've also heard good things about Planet Hollywood across the street. This is prime location on the strip and you have one of if not the best club right in your hotel (Marquee). Also check out TAO too. There are some other spots I went to like Hyde, XS, and Surrender that were poppin but they played dance/electronic music all night so it killed my vibe.
Budget wise, I think I spent around 2.5k but about 1k was on clothes b/c I didn't bring any blazers. That's why I say bring blazers. I went with 5 other people. We split all drinks/tables/taxi rides and the hotel. Gambling too got me a little. But you don't need more than a 1k if you don't drink and don't OD on shopping. Honestly with your hotel/plane paid, you could get by on 800-1000. Plus Marquee entrance is free since you stay at the hotel.
You don't need a rental car. Taxis get you everywhere & the airport is right next to the strip, most owned by East Africans so you might get the habesha hook up.