what would be the cheapest way to lower my jetta about 2 inches
There’s cheap (cutting off a coil), and there’s safe. Check out TireRack.com for lowering springs. It’s about $200 for a set of 4 that lower 1.5". Then you have to remove and replace the struts/shocks, you might replace them at that time. They have a $640 kit that gets you 2", including struts/shocks.
Yes, typically replacing the springs is all that’s necessary. Depending on the car and the drop, you may have to replace other parts in order to allow the car to be aligned properly (i.e.: a camber kit). It may be recommended to replace the struts too.
I did a mild drop on my Acura using H&R springs. I replaced the struts with Bilsteins and also upgraded the anti-roll bars with a set of Eibachs. Finally, I did a Plus-1 wheel upgrade and I got my car exactly how I want it - I couldn’t be happier.