Difficult to recommend just one lens. Anything I'd consider a good focal length would be too wide for reptile close-ups, and anything long enough for the close-ups would be too wide for street. But it's personal, your opinion on the focal length suitable for either or both situations may very.
My guess is that a two lens solution will be needed, something in the 30-50mm range for streetand something in the 85-100mm length for the reptile close-ups. The 50/1.8 is cheap but I personally think it's a litte too long for street/walkabout with an APS-C sensor camera but there are no budget-friendly 30/35mm alternatives I can think of. At the longer end the 85/1.8 is a good value lens. And a set of extension tubes would be handy to reduce the minimum focussing distance for the close-ups.
You already have suitable lenses to determine which focal lengths are right for you on each situation - so why not try with what you have (push the ISO and accept the noise) to see which focal length(s) suit you best. Then report back.