Hey Jyotishvarii,
Hope you got this sorted out. Just a couple comments...
I had the same type situation some time ago. Trying to restore degraded lectures from the tape era.
Bitflipper took a copy of my files and did an excellent job with Izotrope. Requires expenses however.
Saw the reply from Mellatus, and amazed at how much good advice can be contained in a single post.
Without Adobe Audition of Izotrope, the same noise leveling abilities are definitely with the free Reaper plugins as advised. Sonar will pick up ReaFir as a vst extension at no extra cost. As Mellatus said, constant noise is the critical factor. Adobe, Reafir or whatever needs to sample the noise floor. This will require a section of the tape recording, optimally 2 seconds at least to analyze what is to be removed. If there are dogs barking and babies crying, you do not want to use that section or the removal profile will include good data as well.
When applying Reafir, even under the best of conditions, you will lose some of the basic audio. The plugin allows you to back off the intensity of the effect, preview and then commit for the best balance.
Only thing I would add to the excellent expert advice already given.
Once everything is cleaned up as good as possible, you can go back in and add some harmonic content that may have been sacrificed. Artificial, but can make some of the recordings more legible with a bit of experimentation.
Harmonic generators are either costly, or the free ones which narrow onto one band. This may or may not be helpful. Better minds than mine may have better alternatives, but Bootsy's ThrillseekerEXC can rebuild some of the audio taken out by ReaFir. Free and download from here:
https://freevstblog.wordpress.com/2017/04/22/thrillseekerxtc-vos/ Best of luck with your restoration project.
John