Buy the front strut bar, but wait until you can afford the rear bar before you install any of them. As you probably already know, strut bars are designed to keep the upper strut (shock) mount movement to a minimum during hard cornering. The thicker the bar, the stiffer the chassis will be.
They are designed to work with the rest of the suspension mods (i.e. springs, struts, larger/wider wheels and tires, larger stabilizer bars, etc.) so adding just the front strut bar may cause some bad handling characteristics should you get into a panic situation during normal everyday driving.
Again, buy the front strut bar but don't install it on the car until you can get the matching rear strut bar. Then install both bars.
This also applies to high performance springs/struts and the front/rear stabilizer bars. Wait until you get the complete set before you install them on the car.
Be aware, aftermarket tuners make constant changes to their products so I would urge you to wait until you can purchase the modifications as a set.
Good luck!