Faster means that when the lens is set at the largest aperture (smallest number like f/1.2) it allows the most light into the sensor which in turn allows you to use a faster shutter speed. Very useful in low light situations or when you need fast shutter speeds to capture action. Generally, the faster the more expensive, kinda like cars...lol
50mm fixed, or prime means that your lens will be at 50mm all the time. It can not be adjusted. There are markings on your zoom lens. Set it to 50mm and look through it. That is were you will be at with the 50mm prime. There is also a minimum working distance with all lenses. Macro lets you get up very close. That is why they are best for taking very up close shots. A 50mm will not allow you to get up and person but you will be able to crop the photo to get that closeup shot.
Yeah, you will be able to get closer shots further away with the 90mm. That's the only advantage using a 90mm over a 60mm macro lens, it allows more space between you and your subject. That is useful when shooting things like insects that will take off if you get too close. When shooting something like a fish tank, focal length with a macro lens is not that important. Keep in mind though, a macro lens is an excellent prime lens that can be used for a lot more than just macros. Some of my best general shots have been with my 90mm Macro.
For used gear, join up over at www.nikoncafe.com
. You have to be a member 30 days and meet a minimum posting requirement, but it's so worth it as there is a VERY active sales section with great gear all the time. Fred Miranda
is another good one. Finally check www.keh.com
for excellent used equipment. That is all the deal in and they have great customer service and their ratings on the used stuff is very conservative.