If we find a
really good candidate for ET life, there are things other than interstellar probes. We'd probably start by building a much larger space telescope designed specifically for spectroscopy. The funding for a mega-scope would be easier to justify if we had a very strong extraterrestrial biosphere candidate.
There are also some literally far-out proposals to place a space telescope way out beyond the orbit of Pluto and use gravitational lensing from the Sun as a giant objective lens. It wouldn't make sense for a general purpose scope but could make sense if the exact orbit and timing were selected to observe a specific object. Difficult but probably not as hard as interstellar flight.