Sorry but the iTunes terms & conditions state that the internet is required for purchases as far as I can see. It's always required the computer to be authenticated with Apple, that means internet must be accessible.
https://www.apple.com/legal/internet-services/itunes/us/terms.html#SERVICE
REQUIREMENTS FOR USE OF THE ITUNES SERVICE
…
Use of the iTunes Service requires compatible devices, Internet access, and certain software (fees may apply); may require periodic updates; and may be affected by the performance of these factors. High-speed Internet access is strongly recommended for regular use and is required for video. The latest version of required software is recommended to access the iTunes Service and may be required for certain transactions or features and to download iTunes Products previously purchased or acquired from the iTunes Service. You agree that meeting these requirements, which may change from time to time, is your responsibility. The iTunes Service is not part of any other product or offering, and no purchase or obtaining of any other product shall be construed to represent or guarantee you access to the iTunes Service.
If you can't get the internet onto that machine then it is useless for iTunes purchases because the machine is not allowed to access that content. Since Apple dropped 'FairPlay DRM' that may not matter (newer files have no DRM) however those files still seem to want to be authenticated with iTunes as far as I can see - especially for iOS app purchases or video.
Another option you have is to use the device on someone else's computer (that has internet), then try copying those files back to your PC, but you may need to burn them to CD first & then re-rip them (to remove any iTunes purchase tags).
You would be better off sticking to buying CD's if you can handle all the ripping - but without internet access you don't even get the file name lookups - so that means lots of manual work entering all the metadata (that already exists on the internet).