Does it absolutely have to be a combo? Separate modems give you more choice of superb modems, but there is a minor downside which is the MTU 1492 problem which is a weakness in some routers and some modems. If you don't know what that means then I wouldn't worry about it. Some routers have better fire walking more features, give more responsive performance (because of QoS) so that certain activities do not become painfully slow if you have big diwnloads or going on at the same time. Better routers are easier to manage.
Another reason for going separate is that of placement. A modem needs to be very close to the master socket to ensure best performance. You may want to park wireless kit elsewhere though. But as far as wireless goes, separate wireless access points give better performance, are faster and have longer range and freedom of placement provided you can run one network cable to them, which is often a pain though, although wireless-to-wireless distribution (or mesh networking) is possible sometimes, at the cost of performance or eating up more scarce spectrum. Having separate wireless access points means that you can deploy multiple wireless access points parked in suitable locations for superb coverage. The ultimate is - oh help - a three box system, modem plus firewall-router, plus wireless access point(s). It all depends on how much performance and ease of administration you want.
Just presenting the counter argument. There are lots of reasons why a single box may be desirable and only you know your priorities, needs and circumstances.
I use ZyXEL modems times three, a Firebrick firewall-router, an HP switch and several ZyXEL and Cisco WAPs. (The Cisco WAPs promise very high performance but getting software for them has been a ridiculous nightmare.)