D40 lens replacement - One zoom lens? Or one fast prime and higher zoom combo?

Avoid superzooms like a plague, especially if you're ttrying to hone your photographic skills and get more serious. A superzoom would keep you perpetually frustrated with wonky image quality.

Definitely get a nice fast prime and some basic tele zoom (55-200?). You're going to severely miss your wide end, though... An 18-55 is dirt cheap and given the condition, you won't get much for it anyway. I suggest you keep it, even if only for wide angle work.