Nicci, you mightn't be making the mistake, but seeing that you're saying there are
70-300mm f/4-5.6 IS USM going for half the price of a new one (ie around R2.5k) makes me wonder if you're not perhaps seeing people advertising the 75-300mm f/4-5.6 III USM? I have the latter, and while it's a good lens considering how inexpensive it is to learn about telephoto to supertelephoto shooting with, its performance is terrible. You're more likely to get decent shots out of your bridge in a similar range under most circumstances.
The 70-300mm f/4-5.6 IS USM performs similarly but still doesn't have full-time manual focus, so you're paying twice as much for a lens that has IS. By the time you're bothering with that you might as well be getting the 70-200m f/4L USM, which has full-time manual, comes with a hood included that takes a few hundred rand off the price anyway. Has the benefit of not changing in size at all while zooming/focusing, doesn't have a rotating front element, performs very well throughout its entire range almost regardless of aperture employed (many lenses are very 'soft' with a lot of chromatic aberration when shot at their widest apertures for at least some portion of their zoom range).
As for the long-exposures, yes your current lens can do that. Something that facilitates it is the use of a neutral density filter and, if taking the photos at sunset or sunrise, a graduated neutral density filter. To effectively employ both one generally wants to use a filter holder, seeing as you don't want to be sitting there holding a filter in place for as long as 30s - depending on just how smoothed you want the water to look of course. This presents you with a small problem with the 18-55mm kit lens (and the 55-250, and the 70-300, and the 75-300...). Each has a rotating front element for focusing.
You can set your camera to not focus when half-pressing the shutter button and to instead use one of the shoulder buttons at the back - something I do as a rule unless I need to do one-handed shooting - so you'd get your focus using that shoulder button, which will turn the front element, then you can adjust your filter holder carefully, softly, while holding in place the 'focus ring' of the 18-55mm lens.
Another filter that may be of interest is a circular polarizer, though good ones of this, as I already mentioned in this or another thread, are expensive for good ones, and you ideally want to buy one as large as the largest lens you ever intend on owning and using a step-up ring on smaller diameter lenses to fit it. Doing that means you usually can't fit a hood, though - not that fitting a hood is all that practical in many cases since you may need to adjust the angle of your CPL anyway, which involves either taking the hood off or reaching into it to adjust the filter, the latter which isn't possible on some lenses while still looking through the viewfinder to check the effect.
Anyway, with reference to koffiejunkie's last post regarding the USM - not all lenses with USM are automatically fast-focusing lenses. The
85 f/1.2L USM as an example is Canon's slowest focusing lens as far as I'm aware. The 75-300mm f/4-5.6 III USM, as an example, uses a Micro-USM motor which is VERY slow - slower than the 18-55mm IS kit lens your camera likely comes with and slower than the 55-250mm IS USM. The latter which, for its price, outperforms both the 75-300mm f/4-5.6 III USM and 70-300mm f/4-5.6 IS USM while being smaller, light, extends just like both of them do and still has a rotating front element with no FTM just like both of those lenses, too. If you're going to be learning to use your DSLR with a telephoto lens and want to get one right now, you're down to choosing either the 55-250mm IS or the 75-300mm f/4-5.6 II/III (non-USM - the USM and non-USM focus equally quick, both are still noisy, the non-USM one just a bit noisier) if you want an essentially 'expendable' lens for now. The bare minimum I'd recommend stepping up to after that is the 70-300mm f/4L USM or, if you're feeling more spendy, to consider the 70-400mm f/4L IS USM or the 75-300mm f/4-5.6L IS USM or 100-400mm f/4.5-5.6L IS USM, the latter two which fall in a price-range just above either the 70-200 f/4 IS or a 70-200mm f/2.8. The 100-400mm in use takes getting used to and is huge and heavy, but its IS is awesome and it performs very well. Just don't stick a crappy UV filter onto it to 'protect' its front element - it gets streaky bokeh from those really easily.
This is again why I don't recommend just running out and dropping money on things all willy-nilly, and especially don't necessarily listen to the sales guys at camera stores. They're salesmen. The same in large part goes for some of the guys in this thread, as I've been saying all along. One in a store just sold you a tripod for R1.3k - it may be a good tripod, but it wouldn't necessarily have been remotely necessary, or suitable, for what you may end up doing in your first year using the camera, at which point you might even find the tripod doesn't suit your requirements out of one since the head is likely the more expensive component of the kit you bought, and is the part you're most likely to replace.
If I had to play salesman, I'd say the 55-250mm or 75-300mm non-USM for now. They'll last you a year while you learn the ins and outs of working with a DSLR vs a bridge camera and by then you'll ideally be knowledgeable enough yourself that you can research the lenses available to you to find exactly what suits your needs. If you don't need top-tier performance or weather sealing there are lenses like the Sigma 150-500mm f/4-6.3, which costs around R9k and still has apparently great performing IS with full-time manual focus. It even comes with a tripod collar included in the box, which lenses like the 70-200mm f/4L USM don't, because Canon are stingy that way.
If you were to get a lens like that you could get a 28-135mm f/3.5-5.6 IS USM to replace your 18-55mm kit lens. FTM, IS, quick USM and a decent performer, all while costing relatively little for what it is. If budget permits, 24-105mm f/4L IS USM instead, though depending on your requirements out of your photos this might not make sense to get, especially not if you're planning to get it for its weather sealing without pairing it to a weather sealed body to begin with.
Gonna say it again just because. Less money spending now, more learning from free resources, realise you can do a lot of your learning and research for free on your own later and don't need to rely on greasy salesmen to do your thinking for you, and that their thinking is purpose-designed to chase money out of your wallet, no matter how nice they may seem.