If you want coil overs, you will need to build a full new suspension with upper and lower control arms and all of the other things that go along with a coil setup. It sounds like you have a lot of research to do.
What do you need to do? Save up $1000-$1500 depending on your jeep and abilities, not including wheels and tires or lockers and regearing the axles. Read as many SOA build threads and web sites as you can find. Write a list of parts needed for your build and stick to it, pay close attn to axles and steering. Learn to weld, buy a welder, and be mechanically inclined.
A lot more to an SOA than just flipping the axle to the bottom of the springs. Axle upgrades, re gearing the diffs, steering, fab parts, drive line issues, etc. Its simple at first, untill you go to do it, then it can easily turn into pandoras box if your not prepared for what has to be done. The devil is in the details. Do your research and do it right the first time. SOA is not cheap, easy nor fast.