So what's the minimum infrastructure ...
We do not know yet, we haven't did that, and we do not have such systems here on earth, specially for high tech. (because we have not much need to do so here on earth import-export system between countries and cities works pretty good)
Musk thinks about 1 million of peoples, which will be challenge but probably doable.
My estimation are something around 20kk peoples are capable to support infrastructure in the way as we know it now, and develop it farther. (estimation based on sizes of countries, and cities, and implies more relaxed development then Musk suggest)
In both cases we talk about big enough city.
Solar panels - as main part of them is Si (for those where it is, not all) and glass, and some metal(I mean conductors - Al, Cu; frame could be done from glass) - yes we might make solar panels on mars.
As u might see everything isn't so bad. And overall it's expected to be similar with earth compositions, as far as I know, this way some special problems with resources compositions (elements) are not expected. Except gases and water.
But where to search and dig - is open question.
With water everything isn't so simple, there is some, and maybe (not searched for that, but as for analogy with earth) water can be in pockets in the crust, so next blue gold instead black gold is possible and also in deposits of hydrate minerals, besides observed water and ice where this water also should be.
This liquid bears important substances such as lead, silver, tin, bismuth, and antimony. Sometimes minerals in the magma chamber are so hot that they occupy a gaseous state. Others are mixed with water and sulfur in aqueous solutions. The gases and mineral-rich solutions eventually work their way into cracks and become useful mineral veins. Ore minerals, including the incompatible elements, remain dissolved in the hot solution, then crystallize out when the solution cools. Deposits created by means of these hot solutions are called hydrothermal deposits. Some of the world's most significant deposits of gold, silver, lead, mercury, zinc, and tungsten started out this way.
So it's expected to find most things we are using, but where and how concentrated depends on geology processes which had place on mars etc.
Also different asteroids craters, as mars have less erosion could be useful.
But over all there could be so that other processed had place on mars, which could do things better for us in therms of getting stuff. Although we know less then we should.
Plastic and such could be done from CO2 atmosphere, if larger deposits of water will be found.(and that's looks like possibly), big topic https://en.wikipedia.org/wiki/Water_on_Mars
Carbon (CO2) is one thing what makes great difference between Moon and Mars colony, which is vital for self sufficient colony, as we are made from that stuff.