Why is it that everytime I've asked (not on here) about advice on what 'the best' camera/lens is for a given situation I always ALWAYS get the "The camera doesn't matter, a good photographer can take great pictures with any camera... blah blah blah... it's more about to locations and blah blah brag..." But then when we are excited about a new camera it's always "RABBLEABBLERABBLEPOPUPFLASHGIGAPIXELSANDFAIRIES!!!! I can't do my job with this!!!! It's Crap!!! RABBLERABBLERABBLEFILMCAMERAPROFESSIONALSBUNRTHEMALIVE!!!!"
It depends upon the kinds of photos you want to take. If you want to take street photos on a sunny day, an iPhone camera is great. If you're taking photos of fast-moving horses that range from 25 to 300 yards away in good and bad weather, you needs fast glass, flexible focal length, good dynamic range for both intense sun and shaded areas, good ISO performance for hitting quadruple digit shutter speeds on drizzly days, high FPS, weather sealing, durability...
Can you do that with a Rebel and a 75-300?