I've been growing in hydro for about 4 years and just got a ppm meter so no you dont need one but I'll tell you there real nice to know exactly what your working with. There only about $40-$50. Use GH 3 part nutes there cheap and reliable. If you follow this link its a great recipe to use.

http://boards.cannabis.com/hydroponi...iscussion.html

Its a little confusing so if your interested I can give you a simplfied run down of what to do.