Unless your subjects are moving...you don't have to worry too much about low light when using a tripod.
The reason we worry about low light is that it requires longer shutter speeds...and longer shutter speeds cause movement (of camera or subject) to show as blur.
If neither the camera or the subjects are moving...there is no problem.
+1
UE is fascinating but can be quite dangerous. Make sure that you never go alone and that someone on the outside knows where you are and when you are supposed to be out. Then keep to the schedule no matter what interesting stuff you find in there.
Also don't forget to bring a minimum of equipment such as flash lights with fresh batteries and wear work boots.
Have fun.
UE? Thanks for the advice all. I think my brother has a police flashlight, I'll borrow that. No way in the hell you get me in a building abandoned for 50 years without a guide. I have my steel toe boots and my camera battery is a total beast so I think I'll be alright in those regards

. My widest aperture in 3.5.. Will that be low enough? Should I bother trying my flash? Don't even really know how to use it.