I went thru the plumbers union apprenticeship in NY, The apprenticeship part sucks school and all but everyone has to start some-wear go for it. Union work is rarely dirty as in #2/#1 and muck, but standard dirt and grime, Most union work winds up being new work or renovations.
You will likely have to pay for the schooling when I went about 15 years ago it was about 350 a year + union dues. Theirs also certifications you have to pay for like medical gas ect. ect. depending on the area their can be lots of these but most are 50-100$. Some of the certification's are optional but its always a good idea to have them resumea padding in the least.
Union guys can have a lot of unemployment ask one of the reps how much work the apprentices can expect. The guys that are good workers will always be hired first or retained.