The OP need to define a budget for said lens, that would help, second would the OP consider buying secondhand, that would help if the budget limited. As for a wildlife lens, anything from 200mm - 800mm but the bigger the lens the more its going to cost. Zooms v Primes, Primes are better in most cases, sharper faster, but fixed, zooms give you more flexibility. If 99% of the time you're taking images at 300, there a prime lens like the 300mm f4 that would meet your needs, add a 1.4x or 1.7x TC and you'll have extra reach. If your taking images at varied focal ranges, then a zoom probably more to your needs. On a budget sigma or tamron zooms would hit the mark but you're talking closer to £1000/1500 rather than £100. Sigma 50-500mm OS, 150-500mm OS, 150-600mm OS or Tamron 150-600mm, but these ain't fast lenses, but in good light should meet all your needs. And last the weight of the lens, can you handle 1-3kgs of lens (on average) on top of the weight of your camera.