Couple things. A/C units are pretty simple in componentry so make sure it is working right before thinking about installing a new one.
Ice on the lines isn't hurting the unit but can be a sign of high humidity inside your house, which makes it feel warmer, and also makes it a little harder for your A/C to work.
Ice on the coils basically negates much of the work the A/C is trying to accomplish. Again, a sign of high humidity. You may need to shut the A/C off long enough to melt the ice and figure out a way to get the humidity down so that your A/C can run efficiently (without excessive humidity, A/C does remove some moisture from the air).
Make sure that the unit has the correct amount of refrigerant and pressure (this will obviously require a professional visit). An A/C unit will still work with much lower refrigerant levels than it should have, it will just be very inefficient until it stops working.
If after all this the A/C is determined to be working as well as it can then there are likely one of two issues: 1) inadequate insulation or 2) undersized unit.
If at this point I find that the unit is either not functioning correctly and needs replaced, or is undersized, is the point I would go get a new unit.
Goodman will sell to the homeowner. Their stuff is generally used on builder basic new construction. They have improved in quality such that they are generally comparable to the lower end name brand units. However, after pricing out their units, and figuring in my time to install, I called a local vendor and had them install a Carrier unit (only $100 more than Goodman) and warranty their work.
As far as tax breaks go, I'm not sure they still apply. However I know some utility companies are still offering rebates for A/C upgrades - mine was offering $500 or $750.
