Total posts : 45366
Theoretically the bandwidth of a single wire would be narrower than that of a pipe but I have never noticed a difference. Due to the extra work and expense I would start with a single wire and observe what happens. (The Rangemaster, for example, uses a CB whip which is essentially comparable to a wire.)
The dielectric effect of plastic should not be an issue.
There is no need other than for support to solder the wires together at the top since all will be at equal potential at the ends so there will be no current from one to an other.
I once built a coaxial co-linear vertical for 2 meter work and had mechanical problems with PVC pipe. A ten foot length of sch. 40 3/4″ pipe was not very sturdy when supported only at the base.
Maybe a fiberglass hollow tube such as a fishing pole would work better than a pipe.