Take some time to get to know your new camera and learn what sort of photos you enjoy shooting. At some point, you'll start to understand what you need to see in your next lens -- maybe you need more reach (zoom), or maybe you'll end up looking for better low-light performance. Until then, you're really just guessing.
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Given that you've got a camera and at least one lens, any additional lenses you might be contemplating should do one of two things for you, photographically-speaking:
- Let you take a picture you weren't previously able to take
- Improve the quality of a picture you could have taken with your current setup
Consider the first case. If you've got an 18-55 lens, there are wide-angle shots that you just can't get with your lens. Assembling a bunch of photos into a panorama doesn't count. The same goes for telephoto shots -- if you can't frame a far-away object (a bird, for instance) so that it takes up a meaningful amount of real-estate in your image, you don't really have enough reach for that shot.
In cases like these, expanding your focal range with a wide-angle or telephoto lens will allow you to take shots you just couldn't touch before. This case is relatively easy to understand and evaluate. There's a reason you see kit lenses covering ranges like 18-55, 17-85, 18-135, and so on -- these ranges cover the most common focal lengths for general photography. This is the same reason you usually see two-lens kits covering something like 18-250 or 18-300 with a pair of lenses -- that shows you where that next-most-popular range is.
In other words, if you don't have any idea what focal lengths you're likely to need , a two-lens solution covering 18-250 or so is a great place to start. I would expect you to use your kit lens more often than the telephoto lens with a setup like this.
As you become more advanced and learn more about the photos you want to take, you may find that you want to expand even more on the wide or telephoto ends. A wide-angle lens will take you out to 8-12mm or so, which can be useful for landscapes or indoor shots where you don't have room to back up to frame your shot. Longer telephoto lenses (400+ mm) are most often used for wildlife photography. Both of these types of lenses tend to be pricy, and you shouldn't buy one until you know how to evaluate your needs.
The second case takes over where the first one leaves off. There are a number of areas where a better lens would improve on the quality of your kit lens:
- A wider aperture (f/2.8 or faster) allows you to take photos with less ambient light, or to blur backgrounds on photos with a narrow depth-of-field(DOF).
- A prime lens, dollar-for-dollar, tends to be both more clear and wider than a comparable zoom lens. For those reasons, they're very popular for portraits and macro photography. The reason you see the 50mm f/1.8 lens recommended so often for new shooters is that it's a great "bang for the buck" lens that gives you a taste for what a prime lens can do.
- Higher-quality lenses often have improved mechanical bits, including focus motors. Not only does this make them "feel" better, they will usually focus more quickly and more quietly, and they may let you focus manually without having to stop to flip an AF/MF switch.
- Higher-quality lenses will have better optics with less distortion, more clarity, and more resistance to undesirable optical defects like chromatic aberration and lens flare.
- Higher-quality lenses are generally built more solidly with better weather sealing.
When you're just starting out, this is a lot to soak in, and the best way to learn about these things is to go shoot pictures. As you shoot, you'll start to recognize where a larger aperture would be helpful, or why it might be nice to have Full-Time-Manual focusing on your lens. At the point where you recognize why you're want something like that, you've become an informed buyer, and you'll be way better-equipped to evaluate whether you're getting what you need.
In your case specifically, you mentioned both a 75-300mm lens and a 70-300mm lens (in addition to the 50 and the 55-250). When I read your question, it looks like this may be a typo, and that you're really talking about the 75-300 lens exclusively. This is an important clarification, because the 75-300 lens is very inexpensive (matching the price you indicated in your question) and not very well-regarded in terms of image quality, whereas the 70-300 is newer, pricier ($500 new, and easily $350-400 used), and has IS and USM. If you can really get the 70-300 for $150, I'd buy it in a heartbeat. If you're talking about the 75-300, however, which I think you are, I'd be much more cautious. Most of the reviews I've seen really pan the 75-300, and the resale market for this lens is soft. This is important because you're very likely to want to resell that lens -- maybe not long after you acquire it.
The 55-250 is a solid beginner telephoto lens (I owned this lens for a while), but $250 is too much to pay for it. I bought mine refurbished from Adorama for $200, and it's still available there at that price. You should never pay more for a used lens than a refurbished one unless you have very specific and reliable information about the use of that lens. If you believe you'll use a telephoto lens, offer your friend $160-175 for the 55-250 - I think you'll be much happier with that lens than the 75-300.
The 50mm f/1.8 is a great lens, and there's a good chance you'll end up owning it at some point -- most Canon shooters do. At $110, that's a good deal, but not a steal. If you know you want to try out a prime lens, then this is a great place to start. If not, you might consider saving the $110 to apply toward a good tripod or flash - both of which would help the performance of whatever lens is hanging off your camera.
