What properties would the ideal material for spacecraft construction possess? Assuming we develop the capability to send a robot to study Gliese 518, or any of the Earth-like planets discovered in the neighbourhood; the spacecraft would need to travel through the Solar System where it would initially be subject to the Solar furnace, and then further out extreme cold (approaching 0K). It would also be subject to interstellar radiation, and perhaps other forms of radiation that could throw sensitive instruments out-of-kilter. If the target planet was possessed of an atmosphere, and the robot was designed to enter atmosphere it would probably also need to be able to withstand a large amount of heat.
What properties would the construction material need to possess to be able to make an interstellar journey? Is aluminium upto the mark?
 A: I'll begin by answering the easier questions. No, Aluminium is not up to the mark. As mentioned in the comments, the spacecraft we create would probably have several stages that are jettisoned at various points in the journey.
However, given that this question seems fairly hypothetical. If we were to design a single-stage craft for interstellar travel to an Earth-like extra-solar planet out of some new material, it would have to have many of the properties you mentioned.
For starters, the spacecraft itself would have to have power generation capabilities, which would provide the heat for it when in interstellar space. However, as you mentioned, being close to 0K, the material would need a very low emissivity across the IR range, there are many white paints that can accomplish that. Additionally, if we assume that the spacecraft will be travelling at a high velocity, the material will need to be strong enough to withstand initial acceleration as well as hard enough to resist the erosion due to interstellar particles (dust, micro-meteoroids, etc.), which will be impacting it equally fast.
Furthermore, interstellar dust can have chunks up to 100g in size. At 0.2c, these can impact the spacecraft with the force of several (40 more or less) atomic bombs. So, the material needs to be extremely puncture-proof, ablative, regenerative, or Adamantium.
Withstanding the solar furnace is not an issue. If the spacecraft is moving relatively fast, it will reach a far enough distance too quick to need to worry about dissipating the Sun's heat. However, when it reaches the new star system, it will have to be able to dissipate not only the heat of that star, but the heat of entry into the planet's atmosphere. The former requires radiative cooling; a high emissivity, which is easily accomplished by dropping an outer shell and revealing a dark surface pointed away from the star. The latter can be accomplish if the material has a high thermal conduction. The heat (generated on the front side) can be transfer to heat sinks in the back and convected away.
As for the radiation along the way, an atomically dense metal should be capable of shielding a decent portion of it. Of course, it is impossible to shield all radiation due to the creation of Bremsstrahlung radiation, so the electronics would all have to be radiation hardened. For more sensitive equipment, a Faraday cage as well as radiation-blocking materials (ice, lead, gold, deuterium) could keep them more or less protected.
Having described all of these properties, I'm beginning to think that Adamantium would be perfect. Indestructible, good thermal conductor, heavy metal...
A: Will bite the bullet: the toughest time for the shell of any spacecraft (we discount the possibility of reentry at an unknown planet with max and integrated heat flux that can vary several orders of magnitude) is at the end of launch from the Earth.
With this in mind, you want to minimize the mass of structural components. This puts a severe constraint on the choice of materials - basically you have to pick the lightest state-of-the-art material available, and to make a honeycomb from it. Whether it is Al, Al-Li alloy or anything else.
There are ongoing studies in the fields of durability of materials under conditions of near-Earth and deep space (starting in earnest with LDEF experiments in the Space Shuttle program), and the answer that you want is simply not there yet - we don't have any means of getting to even 0.1c without colossal expenditure of resources and should not worry about Bremsstrahlung radiation much, so robotic probes will have to slog it for decades and hundreds of years.
At such timescales, spacecraft's structure will be the last part to fail; any moving or rotating parts are the weakest link at the moment.
