Yes, you would need your reversal before your colonoscopy. The flex sig and gastrografin enema is commonly called a "leak test" and is the last step before reversing your ileostomy. Once you pass that test, the reversal surgery can be done. A colonoscopy is usually done one year following your CRC surgery.
After the reversal surgery, the first few weeks can be challenging so a low residue diet is recommended for awhile. Your GI doc should be able to give you info on how to slowly start adding fiber to your diet. Make sure you have a supply of baby wipes, Imodium, Colace, Senokot, and lots of soft toilet paper!
There are several threads on this forum with diaries of reversals. You might read some of them to see different experiences. I'm a few years from my reversal and can eat anything I want, including fruit, popcorn, etc. However, the first few months were pretty rough. If a particular food causes a problem (like diarrhea), wait a few weeks and then reintroduce it. Your gut needs time to get used to digesting different kinds of food again. Also, you might ask your doc about taking probiotics to repopulate the good bacteria in your intestines. That seems to help a lot with digestive issues.
Hope this helps!