If you want a good paint job, you must have A LOT of time, paitence and some tools. You could do it youself, and get a pretty good result if you put in the work. If you take it to a paint shop you get what you pay for no doubt. doing it yourself steps
1. remove old paint (sand 80-200 grit)
2. fine sanding (400-600 grit)
3. prime 3 coats
4. sand primer till glossy without sanding trough (600-800 grit)
5. redo #4
6. paint 2 coats
7. What i did next when i did my tank was sand with 1000-1500
8. Buff until perfect shine
9. Clear coat