Is the 18-250 lens Canon.
I would guess it is not, probably Sigma.
I would put the values at roughly 550D £75, 18-55 £40, Sigma 18-250 £130.
I would expect lower from a private sale. I was recently offered £50 for a 550D and £25 for an 18-55 IS lens.
When yo say nature, does that include birds and small wildlife? The 18-250 may give you a taste, but you would soon want longer.
The 18-55 is OK, but a 70-300 may be a better companion to it. From that is may be better to look at buying the setup separately.
On your stated budget, the 550 is probably as good as you will get. The 650D is basically the same with a swivel touch screen, and the 800d will use you budget up on just the camera.
Only my feelings on it, I changed from Canon to Panasonic M43 about a year ago