But was it actually necessary to buy Windows-specific tools?
As usual it depends on the value of _'actually necessary' used.
Strictly all needed was a linker producing NE (windows) binaries. Ofc, when calling Windows functions (hard to avoide) it's calling convention need to be satisfied. Soeither some wraper libraries where needed or low level code had to be added to do do the calling, or just change your style to do so (as I did with assembly (*1)).
If you programms where already nicely modularized (read, use external symbolic addresses for linking), switching to Windows wasn't a big deal.
*1 - Jup, my first Windows programms where Assembly - as most stuff I did :))