Looked at the NX-5300 very interested but programming software hard to find
… as in not freely downloadable from the interwebs? Kenwood will be happy to sell it to you.
And Kenwood has done that on purpose. Each copy of the software for these newer radios gets tied to a specific computer via a piece of licensing software and a unique key from Kenwood. You might find the software on the web, but you won't find a valid key you can use.
If bootlegging software is your plan, you'll need to stick with the older radios.
Start asking around with local shops. I had one of the local shops take a bunch of older Version 1 TK-5210's on trade in for Version 3 models. They were selling the 5210's to the right customers for $100 each.
Unlikely you'll find one that cheap, but asking around is a good start.