Gatorgal,
A telephoto lens with a "macro" setting is quite good. I have a Nikon f55 with both a 28 to 80 lens (Tend to use for large Monitors, Crocs etc) and a 70 to 300 telephoto with a macro stting with a ratio of 1:2. Its max enlargement is something about the size ping pong ball at the distance of about 75 cms from the subject. As with most herp photography you want a high F stop...and with a large lens you need light...I use the SB 24 speed light. In using a set up like this you do not need to be to close to any species and such do not have the problems that come with being extremely close the the subject....Mainly lighting issues.
I will make one suggestion however, I you are not comfortable AND able to handle the subject you are photographing even a distance as mentioned above, is not safe for photographing dangerous herps.
Regards,
Scott Eipper