Earnest Lad,
Ditto what Marc said, that manual focus lens you linked is a novelty at best, kind of like drilling your own holes with hand tools, or making your own wooden nails. Steer clear of it.
The Sigma 150-500, or 50-500 can get you into it, but really you'd be better off finding the small amount of extra folding to get the Sigma 150-600 Contemporary. This is the minimum level I would recommend going to for the lens. This will get you to 900mm equivalent. As Marc said you are better off putting your money into better glass than a better body. The Tamron 150-600 G2 is your next step up after that, though expect that to be a few hundred dollars more. After that you are into the realms of exotica.
The Nikon D7100 is a great value body point too. Only the 7 series bodies from this one on will give you the 1.3x in-camera crop feature (not available on any of the 3 and 5 series). This will get you to 2x, or 1200mm with a 600mm lens. With the D7100 it helps a lot since it reduces file sizes and gives you an extra fps speed to take it up to 7 fps. The D7200 body is very slightly better again because of it's bigger buffer. Both of these models are discontinued. Only the D7500 is current which shouldn't be too much more and gives you a much improved resolution metering sensor which helps AF tracking, and 8fps which is great fun.
If you must save on the body then you will pretty much be using Centre point focus only, so I don't think it matters too much which series you get - 3 or 5 - choose the one you can afford and that balances well with the Sigma C (though I think best balance will be achieved with one of the 7 series cameras [D7100, D7200, or D7500]). The sensor resolution and IQ of the 3, 5, and 7 series is all pretty similar.
*IMPORTANT* If buying the body second hand (especially if superceded and discontinued), check the compatibility with the lens on Nikon's website and the lens manufacturers website.
I think the advice you have had is about as good as anyone can give. It's up to you to find the moolah now
.The Sony route (native lenses) is likely to be outside your budget. The new Canon 90D body ($1199 available mid September) certainly has high resolution (32.5MP @10fps), but early sample shots indicate that it will need expensive glass to perform well with sharp shots - I'm not even sure the 3rd party superzoom telephotos will prove up to it that well. The Siggy/Tammy 100-400's will likely do a little bit better, and the Canon 100-400 f5.6L IS II most certainly will, but then you are giving up reach (only 640mm equivalent, though it is likely to crop readily to at least 800mm eq). The shorter Siggy /Tammy are likely to be in the ballpark of the cost of the 150-600's anyway, though the Canon L lens will be nearly double that, so I think that puts it out of the budget range too .....
Good luck !
Chosun :gh: