1. Talk to your isp. Some ISP will require you use their modem. Some will try to rent you a all in one router/wireless router. Be sure if possible to get strictly a modem.
2. The router you get to will depend on your budget. Wireless AC routers will be the best option but they may mean upgrading current wireless cards in order to get the most of it. Wireless AC is backwards compatible so you won't need to upgrade devices that don't need a super solid connection.