I have two myself, an I1, and I2 (hydro magic). I love them, and tend to use them when I'm a little pressed for time. The result is always an effortless DFS, which is what I shoot for 95% of the time.
P.S. I forgot to mention I paid about $15-$20 for them not long ago, both in great shape and with cases, just for a reference point.
The injectors are great. And, yes, you need one. I think the "G" is a good all-around razor and a great place to start. Very common and dirt cheap. I believe I paid $6 for mine including shipping. No case at that price. Post a note on the B/S/T. No doubt you'll have lots of offers.
BTW, if you do get one, pick up some of the Chinese-made Schick injector blades (Chick blades) from Amazon or one of our local vendors. The Chicks have a better rep than the Personna injector blades.
I paid $6 from a local antique mall for my J1. Blades came from ebay, $8 for 7 including delivery if I remember correctly. I keep mine in a locker at the gym and use it for a quick 12 hour clean up when ever I'm there. I get a good shave, but it's just not as satifying as a DE razor. Usual YMMV disclaimer applies.