Two fundamental components to the modern DSLR camera - the body and the lens.
I've always been a firm believer in the following philosophy.
Your money is better spent on a quality lens than it is a new camera body.
There's a few things as to why spending your money on lenses is better than a camera body.
The same lens you've just bought you will still be using in a year, 3 years, 5 years and in some cases ten years. The best lenses hang around for decades. You'll be throwing your DSLR camera out every year as the new one comes out. Not the lens though. Its work of art.
The lens in most cases holds monetary value. Most lenses I've bought I've sold for the same price. No skin off my nose. I've got use out of it and it's essentially cost me nothing.
The research and development that goes into lenses is very extensive. They have to last. The market has this has this toxic addiction to getting new camera bodies with more megapixels, blah, blah blah, released every year. From a marketing perspective it sells to develop camera bodies and thats what the companies exist for - profit for shareholders.
Rarely does a lens break or wear down to a point where it is no more use. They are an investment.
The telephoto lens is most certainly the pick for all spotters. Our hobby requires us to photograph from a distance and so to cut down on distance you need a telephoto lens.
Canon have their famous L series range. In my books the L stands for Luxury. Truth be told I don't know actually know what the L stands for and I don't really care. These lenses are Canon's best. These lenses are the investment I've been talking about.
A few quality lenses come to mind - Canon 100-400L , Canon 70-200 f2.8 L II and if the budget stretches Canon 200-400 F/4 L.
All of the above lenses are quality!
To get that little bit more though you are still left with the Primes. The prime lenses are fixed and thus they are fast lenses. They have far less vignetting than zoom lenses. Typically most of the zoom lense I mentioned get soft at the edges. The prime lens is sharp right to the very edge. If your budget stretches this should be your choice. No if's, no but's the prime is the ultimate weapon in aviation photographers kit.
You pay for what you get.
I cringe so hard when I see people with top end DSLR's out spotting and they've got some kit telephoto lens put on the front of it.
My advice is always the same - buy the cheapest DLSR body and spend your money on quality lenses.