If you are on a tight budget buying into the FX system may not be the best route. If you are using a DSLR of any kind you aren't going to be sneaking up on anyone. If you want to sneak up on people get the Fuji X100 and forget about SLR lenses altogether or Lecia M9, and you think Nikon FX is expensive! Also, there is no need to rush out and get all your lenses at once, pace yourself!
As for why the AF-S primes are big compared to manual focus lenses, are you really thinking this through? The focus motor and weather sealing is what is taking up the space, not to mention far superior optics (other than CA which is just the price you pay for having fast primes). Also consider ergonomics, on the FX cameras, which they are designed to be used with. They might seem huge, but mount them on an FX body and they are very comfortable and a pleasure to use.
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