A TC will do two things which are bad :
It will reduce the effective aperture by a stop (for a 1.4x) or two stops (for a 2x). For wildlife this is an issue as you start battling for shutter speed a lot more often.
It will generally reduce sharpness and can add some CA. But this reduction is not really bad.
From my own experience I see no real advantage to using a TC compared to cropping and enlarging. I've see some detailed tests, but these don't convince me that there's much difference between a crop and TC. Maybe a slight advantage for a TC with a good lens, but it's slight.
As @dan-wolfgang suggested, the 55-200 or the 55-300 used or new would be a better choice on a budget.
And I've used the 18-105 and 55-200 and they're both good lenses. I'd never use either with a TC. I've used TC's in the past and have long ago discarded them as a nuisance. YMMV but for me they're a poor choice for the money.