How far could we go towards engineering a space-durable human species?
I think this question is likely to get closed as off-topic. It is extremely hypothetical and would be a better fit on WorldBuilding.SE. But here is my messy attempt to answer this question.
So, I guess in your question, you assume that we know everything about how our genotype (≈the genes of an individual) make up the phenotype (≈how we look basically). So, I will try to give a short answer to this question by assuming that we were to know exactly what would happen if we were to add the sequence
ATTTAG in the somewhere in the alpha-globulin gene or the sequence
CCTATA somewhere in the regulatory sequence of the lactase gene. It is important to understand that today we are very far from having such knowledge. I also assume that we are able to recreate the environments we need to develop the new thing (human2.0 if you want) that we would have engineered. For example if we make up humans that need to grow in a different environment than the womb. By the end of the answer, I will pretty much assume that we know everything that can be known about organic chemistry and how chemicals process genetic information. Note that to the point we're doing the drastic modifications I will be talking about, it will be a philosophical issue to decide whether the new species we created is a human or not (it may even come to the discussion whether the new thing we created is a living thing or not).
Resistant to evolution as a species or only resistance as an individual?
Are talking about a species that would definitely be resistant to survive in these extreme environments or are you talking about a bunch of individuals surviving in this extreme environments. The difference is "evolution". Species evolve and not necessarily in the right way. High radiation may yield to high mutation rate and therefore to mutational meltdown (decrease in mean fecundity of the population through time yielding to extinction of the species).
With these very extreme assumptions, then we are only limited by physics. We could not add wings of the size of the wings of a warbler to a big animal like human and hope that it is going to fly.
Resistance to radiation
I know little about this subject but will try to say some things that may eventually help you. We cannot really make our DNA insensitive to radiation (although we may eventually make our DNA slightly more stable) because radiation will alter matter. It is a physical process that cannot be altered. We may create a lot more of tumour suppressor genes to protect ourselves from developing cancer. Eventually we could create a species that would be able to synthesize (and carry) a concrete shield around its body to protect itself from radiation. But it might eventually be easier to code genetic information with something else than DNA! Eventually made up of quartz because quartz is relatively insensitive to radiation. We could indeed create a whole new form of life.
High radiation yield to high mutation rate and high mutation rate may likely lead a species to extinction (mutational meltdown) but not necessary directly kill the first generations of this made-up species.
This sounds "easier". We may "just" target the alleles (=variant of a gene) that are known to diminish the human lifespan, to slightly (but probably considerably) increase human lifespan. Again, I assume that we can know these alleles and can replace them with better variants. However, evolutionary processes will just recreate those genes, just because there always has an equilibrium between the rate at which new deleterious mutations appear and selection purging the genome from these mutations. Moreover, at an age where reproduction has already occurred selection is less important and it allows for deleterious allele that are expressed late in the age of the individual to eventually spread in the population.