Don't pretend that you can scan slides in a flatbed scanner. If you want to do slides, get a dedicated slide scanner. For other tasks, get a flatbed or sheet-feed scanner, as appropriate. That said, one of the Nikon CoolScan products would seem to be at the knee of the quality / price curve. If you need to squeeze every last drop of quality out that the Nikon would miss, then you can shell out, per slide, on professional drum scans (but only for the ones that you actually care about).

With respect to PhotoCD, I paid $1/frame for these things for everything I shot from about 1993 to 2000 or so, when I switched to a digital camera. When processed by Kodak, the quality control was piss poor. I was always putting elbow grease into dust spots, water spots, and so forth. There used to be third-party labs that charged maybe $1.20/frame that had better quality control. PhotoCD always had a sort of "haze" about it, but you could easily work around it by adjusting your levels. If I was to shoot film again, I'd probably shoot slide film and see if I could get it processed and scanned all at the same time. The price is high enough that it's pretty hard to justify relative to current digital cameras...