First be aware that Canon is gradually phasing out DSLRs and the lenses for them (like yours) and replacing them with their new R-series mirrorless cameras and RF lenses. Even so, there are A LOT of EF and EF-S lenses around for use on your camera and there will no doubt be both those and DSLRs to use them upon for many years in the future. But, eventually you should plan to transition to the mirrorless system.
That said, the EOS 100D (SL1) is a fairly basic, entry-level camera. The 200D/SL2 and 250D/SL3 saw quite a few improvements. They got a new sensor with higher resolution and greater dynamic range, supported by newer processors that make both cameras able to take a lot more shots per battery charge, as well as faster frame rates and better overall performance. The SL2 and SL3 also got some other nice features like an articulated rear LCD screen and a self-cleaning sensor. The SL3, in particular, saw additional video-related upgrades. Something that wasn't improved across these three models is their AF system. It's essentially the same and remains a pretty basic 9-point system that is similar to what Canon was offering on cameras in 2004 or 2005. You would need to step further up to a T8i or 90D to get significantly better AF system.
BUT, honestly you have a capable camera. The differences between it and the newer models aren't that big and you would far more likely open up new photo opportunities with one or more additional lenses or with lens upgrades. But the lenses you choose depend A LOT upon what you want to shoot.
A 70-200mm lens is popular among sport and wildlife shooters, as well as photojournalists. Canon has made a whole bunch of them, with various. However, if you are looking at an f/2.8 model that's selling for around $600, that's likely the oldest and least capable model, which is still in production (and sells new for over $1300). It and all the EF 70-200s with f/2.8 aperture are fairly large and heavy. About 3 lb. You will probably find they feel unbalanced using them on the smallest and lightest DSLR anyone has ever made, the SL1. I have lent my Canon EF 70-200mm f/2.8 to several people to try out and they have chosen another lens due to the size and weight.
If a 70-200mm is what you want, I'm going to suggest a different lens; Canon EF 70-200mm f/4L. There have been several versions of them, too. They are about 1/3 smaller and lighter than the f/2.8 versions. Like the bigger lenses, the f/4 versions are pro-quality... very well built and good performing.
Another feature to look for on these lenses is IS or image stabilization. There are less expensive models without it, but it's a great feature that I highly recommend. There have been three f/2.8 versions with IS and two f/4 versions. The latest and greatest are the best, of course. But going back one generation and buying used you still get a superb lens with the EF 70-200mm f/4L IS USM. [Removed 3rd party link per Community Guidelines] These lenses normally come with a lens hood and can optionally be fitted with a tripod mounting ring. (Canon Tripod Ring A W... Tripod Ring A-2 or A-II W will also fit and work, but is a brighter white that doesn't quite match the lens. There also is a cheaper Vello version that may work well.).
The biggest problem with adding a premium quality lens like the EF 70-200mm f/4L IS USM to your kit is that you will probably no longer be happy with the kit lens you got with the camera, which was okay for a starter but can be improved upon. If you decide to replace that too, I would recommend a Canon EF-S 17-55mm f/2.8 IS USM. It';s available new for upwards of $800, [Removed 3rd party link per Community Guidelines] This lens does not normally come with the EW-83J lens hood it uses, but sometimes is included when you buy used. That hood is sold separately for $49 new (but there also are less expensive 3rd party versions such as Vello).
Overall, I would encourage you not to get too caught up in the gear and to spend more time out shooting. Keep your kit to a minimum (unlike me!🙄) and learn how to get the best out of it. Depending upon what your shoot and how you shoot it, things like a good tripod or flash or nice camera bag might be as valuable or more-so than a camera upgrade or added lens. How-to books, post-processing software and monitor calibration tools are also good investments you might want to consider sooner or later.
Enjoy!
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7DII (x2), 7D(x2), EOS M5, some other cameras, various lenses & accessories
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