I really don't want to shell out for a scanner then just have it laying around after I'm done scanning my old photographs. Also for the purest I would really need a good flatbed ($200 or so?) and a ...

With 500+ slides at a guess based on scanning my own film negatives/slides assuming you can do 3 slides (might be 4, maybe 5 depending on scanner) and 15 minutes per scan something like 42 hours of ...