Just some general information... You can get more specific details, when you decide to get a Mac.
Your external drive is probably formatted as NTFS. Macs can read that format, but it is read-only (for the purpose of transferring data to a Mac). Since 5TB is larger than the size of a Mac's internal drive, you'll need to get another external drive that can hold (at least) your iTunes data. Macs can use any standard USB external hard drive, or faster ones that connect using the Thunderbolt port (depending on the Mac model). When you connect it (the new external hard drive) for the first time, you can "erase" it (using the built-in Disk Utility app) to reformat it for Mac, if it is not already formatted for Mac.
Since your external drive is so large, as a first step, you should copy ALL the data you wan to keep to the new Mac-formatted external drive. You can later reformat and "repurpose" the old external drive as the Time Machine backup archive. The extra-large capacity makes it ideal for this purpose. So, getting a new external drive won't be like wasting money. Time Machine is a built-in feature of OS X that keeps your files backed up automatically, every hour that the system is running. If you have a data issue, you can recover specific files or the entire system to the latest backup state, or to any previous saved backup state. Time Machine can back up the Mac's internal drive and external drive, to (another) external drive.
Then, transfer the iTunes data from your Windows PC to the Mac. How you do this depends on how iTunes is currently set up on the Windows PC. If you have your complete iTunes folder (with the iTunes Library database file) on the external drive, copying it to the Mac-formatted external drive does the transfer. If your iTunes folder is on the internal drive of the Windows PC, and only your iTunes Media folder is on the external drive, there are more steps involved in transferring it to the Mac.
The key is to transfer your complete iTunes library to the Mac. If you just transfer your iTunes media files and add them to the Mac's NEW iTunes library, that may actually help resolve your current issues, but you would also lose all of your supporting data, such as playlists, ratings, play count, etc.
