Been thinking of getting into the hobby of building ultralight helicopters, and would PVC tubing be a good choice to build the overall airframe?
No. The strength to weight ratio of PVC is poor compared to aluminum and you would end up with a very heavy, albeit cheap, machine, and will be even worse when compared to carbon fibre tubing, which would be the optimal choice performance-wise. Extruded aluminum tubing would likely be the best choice cost wise. Forget about building a plumb-O-copter.
PVC is no where near stiff enough for a given weight. And its strength to weight ratio is poor. The structural efficiency of air machines is absolutely critical otherwise you are building a machine that may not fly at all.
Plus, do a search for helicopter ground resonance, especially watch some youtube videos of helicopters destroying themselves.
PVC pipes are strong and flexible. 1 1/2" diameter pipe type SDR-26 is thinner-walled and weighs only .3 pounds per foot! That's three pounds per ten-foot length which only costs eight dollars each at Home Depot. PVC cement can form a solid integrated bond when applied thickly and left on for ten seconds to melt the surface before mating.
However, PVC is breakable when bent, so the airframe must have a lot of reinforcement braces to reduce the amount of bending of longer sections.
Strength-TEST.!! Test test, the strength of every section using sandbags. Get an EAA member to advise you on this.
Regarding self-bendability: I just tested a 10' length of 1 1/2" PVC type SDR-26 plastic pipe and when suspended it at center and each end drooped down 1/2" is all. I doubt that any ultralight designer would incorporate any unbraced pipe lengths longer than 2'.