May as well replace all the o-rings while you have the system open. If you don't buy an o-ring kit, pull the o-rings and match them at your local auto store to get an exact match. Also, make sure you use two wrenches for each joint (where you see two opposing nuts). You need one wrench to hold and the other to turn. Make sure you don't over-tighten the joints. You want a tight seal, but over tightening can ruin the aluminum threads, leaving you with a bigger problem.

You'll also want to carefully clean the evaporator coil. I have an '85 and not sure about the specifics on your model, but coils tend to clog up with dust, reducing air flow.

Flush is necessary, make sure you use a compatible oil with the refrigerant you're using. Also, do change out the receiver/drier. Most compressor warranties will be voided if you don't take this step.