time capsule as a nas AND wifi access point

The Time Capsule hard drive acts the same no matter whether the WiFi is active or not, so the Time Capsule could act as a WiFi access point if it is connected back to the main router using a permanent wired Ethernet cable connection.

Apple discontinued the Time Capsule about 3 years ago, so best to proceed carefully if you are considering the purchase of the product. All Time Capsules are out of warranty at this point, and Apple offers no repair services or parts for the product.

is there a way to make it work as an access point without having to be connected to the main router?

Yes, if your "main" router is also an Apple AirPort product. In that case, the Time Capsule can connect using WiFi back to the main router. The Time Capsule will need to be no more than a room or two away from the main router to work in this type of setup, since the WiFi signal strength declines quickly with distance and obstructions.

Unfortunately, there is a real downside to this type of connection though, and that is that you will lose at least half of the potential speed on the entire network when the Time Capsule connects back to the main Apple AirPort router using only a WiFi connection.

So, while your network will have greater range, it will be a slower network. You can avoid this painful speed loss if you connect the Time Capsule back to the main AirPort router using a permanent wired Ethernet cable connection.

If your "main" router is a non-Apple device, the only way that the Time Capsule can act as an access point is with a permanent wired Ethernet cable connection back to the main router. It cannot connect to the main router using only a wireless connection if the main router is not another Apple AirPort product.

MoCa adapters or powerline adapters are a possibility if you cannot run the Ethernet cabling, but neither of these types of connections will perform as well as a simple Ethernet cable connection. But, they are a better way to go though than trying to use only WiFi to extend a network.

Makes no difference what model that you get. The potential speed of the existing network will decline by half or more if you try to connect the Time Capsule using only a WiFi connection.

Frankly, it would not be wise to even consider a 4th Gen Time Capsule since the product will be more than 7 years old as as much as 10 depending on when it was placed into service. The average useful life of a Time Capsule is about 5 years.

Even if you go for a 5th Gen Time Capsule, the product will be at least 3 years old and as much as 7+, so you would want to come up with a plan to back up the data on the Time Capsule to another hard drive on a regular basis.