Replace S-RAM S7 gear hub with something better
The cheapest option is probably to buy a whole new wheel off the internet. The Sturmey Archer 5 speed with drum brake is my first choice (www.sturmey-archer.com/products/hubs/cid/4/id/33) - costs about 140 pounds for the hub from SJS in the UK. The advantage of the Sturmey hub is that it's designed to work in various frame spacings so you just remove a couple of spacers and it'll work. They're rumored to be quite robust but I haven't tried the 5 speed, only the 3 and 8. But the brand is good (was UK, now Taiwan, which seems to have lifted the manufacturing consistency a bit).

If you can use a disk brake on your bike the Shimano Alfine 8 is a good option, about $US400 with a cheap brake by the look of it. (do not get a Nexus hub, they were a bit of a learning experience for Shimano). They're available with a disk brake as an option, so it comes down to what your bike frame will accept. If it's got an internal brake now it'll almost certainly accept the drum brake, but if it'll take a disk the Alfine is a slightly nicer hub.

With the Alfine you'll probably have to spread it to 135mm, as (from memory) there aren't any spare spacers you can remove. The SA with drop right in. But if your frame is steel you will almost certainly be able to just spread it the 5mm to take the slightly wider wheel. If you're not comfortable doing that a bike shop or framebuilder will be able to "cold set" (ie, bend) it for you. With aluminium or composite frames that's not an option.

(I don't have enough points to embed the third link, sorry)