Look @ the canopy of the tree, multiply that by 2 and that's usually how far the root system will reach.
I've personally (with my father) dealt with a few older homes with lots of trees. As others have mentioned, quite common for the incoming water or the clay drainage pipes to break. Can be very costly to repair unless you wanna spend lots of time digging (which is what my first hand experience was
)
Personally if I was buying a townhome I'd stay away from anything that may have problems in the future because I imagine it being a huge hassle if you did need to make repairs and split the cost with a neighbour.