Here is the jest of this article. Please don't think you need the most expensive gear to get good results and enjoy outdoor nature photography, your results are more related to you, your attitude and ambition than the cost of the lens and camera. Get out with what you have and enjoy the hobby to the fullest!
Although I am a Nikon user the recommendations apply equally to any DSLR shooter. So, I ask myself if I woke up one day and all my gear was gone and I only had $2,500 to spend what would I get?
This will be a short write up because technology has brought us to a place that has available very good super zooms and fantastic DSLR full sensor or crop bodies at reasonable prices. I predict the trend will be for companies to introduce more reasonable cost effective quality lens and cameras due to the hand held world of smart phones photographers.
One topic that becomes rather tedious is what is a "low cost" system? and that is a good question that can not be defined among the many varied budgets out there. But the reality is you either have the cash for new or search for used gear and at the end of the day it matters not. For the article I assume you are willing to spend up to 2K.
The Lens: 100-400mm, 200-500mm or a 150-600mm (Tamron, Canon, Sigma or Nikon), As a Nikon shooter I would go with the 200-500 since it is a constant aperture of 5.6, even though it is slightly shorter than the Sigma 150-600. For Canon & Nikon Sigma is offering a 150-600 F5-6.3 DG OS for less than a grand, pretty sweet.
The Bodies: Nikon D7500 or D7200, Canon EOS Mark II or D80S; Also is the options to search for used gear, I would try the BandH or Adorama sites, as they have high reputations for standing behind what they sale.
For me my kit would be a D7200 and 200-500 with enough left over for a few memory cards, otherwise I would find a used 200-500 and pick up the newer D7500.