Thank you all for your input and explanations.
So essentially, a 'pro' body and lens could take very similar quality photos to what my D3500 and kit lens is capable of, it just is a lot more durable with things such as weather sealing and more reliable?
I see that the pro models also have things like more focus points and dials to change settings quicker, instead of diving into the menu like my camera.
I could of done with weather sealing earlier. Went for a walk with a friend intending to take some photos, and the heavens opened. I covered my camera the best I could, it did get some water on the outside that wiped off. Was a bit worried but it is working fine.
It was only a thought, I was thinking if I progress enough to be able to make money from my images in the future what I would need.
Would a mid range Nikon be suitable for 'pro' use if it is weather sealed? I am using the DX format currently and am impressed with the image quality, so in the future maybe even one of the 'pro' DX format cameras would be good?
No, a good quality, pro-grade DSLR will usually do an awful lot more than an entry level DSLR. The results may look similar on a nice sunny day in 'ideal' lighting conditions. but the pro camera will usually produce much better results in more difficult lighting conditions. Then there's the functionality; a pro quality DSLR will usually give far more control over the various 'settings options', it will usually also offer numerous custom function options, where the photographer can pre-programme the settings they require to cope with various lighting and shooting scenarios.
Then there's the speed at which the camera saves and buffers the shots taken, once again, the pro camera will usually be a lot quicker than the entry level camera. Then there's the lenses; pro quality lenses will usually be 'faster', allowing more light to pass through them for the given focal length. They'll usually give less distortion and chromatic aberration, and may well offer 'image stabilisation' of at least 4 or 5 stops.
Then there's the compatibility with accessories such as flash units, power boosters/grips, etc., etc. The pro camera will usually offer significantly more expandability and integration than entry level kit.
Put simply, within reason, you usually get what you pay for. Yes, it's entirely possible to get professional looking shots from an entry level DSLR (or indeed, an old film camera from the early 1900s), but a pro quality camera should give a pro photographer a significantly better chance of getting a good result, just as a high-tech works rally car will give a pro rally driver a much better chance of winning a world championship rally than an ordinary family saloon.
As for weather sealing, it depends if you actually need it. Can you tuck your camera under a waterproof coat instead if the heavens open? Or, perhaps, avoid days when the weather is bad? If your style of photography doesn't demand weather resistant kit, then could the money you save be better spent on another lens, a higher-spec flash unit, etc.? For instance, how many Formula 1 racing car teams spend money on snow chains or studded ice tyres?
Pro photographers tend to spend money on camera kit based on actual need, and the existence of a sound business case for doing so; they will usually only buy stuff if they are fairly certain that they will actually benefit from the investment. Perhaps amateur photographers could learn something from them in that respect?