A spacecraft at the Sun Earth Lagrange point one (1% of an AU, or one Solar diameter, from Earth towards the Sun) would have the Sun as its background. Is the Sun emitting strongly enough at wavelengths otherwise ideal for communication, that it requires important design changes compared to spacecrafts in other orbits? What wavelength would be ideal from SEL-1? What are the challenges, if any, and solutions to discern data from Solar noise at SEL-1?
A spacecraft at or within a few degrees of the Sun-Earth L1 point would be unable to communicate with the Earth due to interference from solar radiation. Moreover, antenna operators very much do not like having their antenna pointed toward the Sun. Similarly, a spacecraft at or within a quarter of a degree of the Earth-Moon L2 point would be unable to communicate with the Earth due to blockage by the Moon.
Even without those communications concerns, spacecraft do not operate at the unstable Lagrange points. They instead fly in some kind of pseudo-orbit about the desired Lagrange point. One reason is stationkeeping costs. A spacecraft in such an orbit needs to perform a small number of smallish stationkeeping operations per year. Moreover, such a spacecraft doesn't need to know where it is. Those infrequent stationkeeping operations can be calculated on the ground and uploaded to the spacecraft as a delta-V maneuver to be performed at a specific time.
In comparison, a spacecraft operating at a Lagrange point would need to perform much extremely frequent stationkeeping operations, and its flight software would need to know where the spacecraft is in space. The orders of magnitude higher stationkeeping costs combined with the more complex (and hence more expensive) flight software preclude satellites from operating directly at any of the unstable Lagrange points.