If you're going Plustek, I'd suggest looking for a 7500i or a 7600i if you can; they both have the infrared channel (AKA ICE) which does work well for C41 (inc C41 B&W) and E6. If you have the need and the patience (and old, seriously dusty material) you can get a surprising amount of value from the IR dust removal for Kodachrome and presumably also black and white. You just have to use the masks in Silverfast... which is better than useless, but definitely a PITA and very un-intuitive and hard to use! Vuescan Pro is much better, ad it's buy once, use with any scanner. (SilverFast is tied to the scanner you bought it with!) Oh, from my experience if you're scanning newly processed material from labs, you'll rarely have much of a dust or scratch problem...
But if there's a reasonable prospect of 120 in your future, buy a V500 now and save yourself getting one later! And V500 35mm scans are certainly good enough for any web use, and for pretty darn good prints too. The difference is mainly for pixel peepers, and filmies wouldn't do that, would we?