The stouter the valve springs the harder it is to compress. Do wear some eye protection though cause if you happen to slip off the valve retaning clips have a habit of becoming airborne (Did that once many moons ago.had a hell of a time trying to find them). Before I bought mine I looked at one at Sears, it wasn't near as sturdy and the price was about the same. I believe the plastic retainers you are refering to are the oil seals over the valve guides and if they've been on there for any lenght of time they'll probably break into pieces when you remove them. If you don't have a new set you should be able to get them at a good auto parts store.
Marshfield is a nice little town and I like the KC area, just too congested and too damn cold in the winter for me.