What an exciting project! It's so much fun to get something back in use.
My greenhouses don't have glass panes so I'm probably not the best person to answer your question but I think if you don't fully replace and reseal the panes the remaining stripping will continue to fall apart and eventually the glass panes will become loose and be at risk for falling out and/or letting weather, insects and rodents into your greenhouse.
But if you're going to go to all that work you should probably first assess whether the existing panels are worth saving. Depending on how old your greenhouse is you might want to replace the panels with new glass or polycarbonate panels. Many of the newer panel products provide superior insulation and performance compared to older products.
As to what you should do to get your greenhouse ready I'd say (beside fixing the panels) first you should clean and repair the inside of the greenhouse. When your greenhouse is in use it will be hotter and wetter than it has been since it went out of use. So you'll want to paint, repair, seal, etc. I assume you have a drain in the floor somewhere. I'd make sure that is working properly. Check that your electricity is working. Is there a heat source? Make sure it works properly. Make sure the vents work. Consider adding additional venting if you think its needed. Are you happy with where your water source its located? Once you fill your greenhouse with plants all those repairs will be much harder, so its best to do them now.
Think about what you want to grow and when. A setup to grow orchids year round is very different than what you would need to grow a few tomatoes in the summer. Do you want to grow in beds or pots? Maybe even hydroponics? You'll probably want some plant racks and you'll certainly want storage for tools and supplies.
Once you know what you want to grow, come back here and share your plans and we'll help you get going.
And if could please tell us where you live, what direction the greenhouse faces and how much shade it gets.
