Q: Windows on Mac.

I will assume you are speaking of the windows OS...  I have used a virtual drive and bootcamp and found that bootcamp works the best.

How much RAM do you have?

There are are several options that allow you to set up and work with virtual machines. You might also be able to use Crossover, although the last time I checked what Windows applications worked with it, they were a couple of versions behind the current ones.

Parallels 11 or 12, or VMware Fusion 8 would allow you to set up and run virtual machines. You would also need to buy a Windows OS to use with those apps. Virtual Box is a free option but it is said to be trickier to set up.

Rebooting is necessary only if you setup Windows on separate partition, as Boot Camp will do.

Windows isn't free. You'll have to buy a full version license (not an upgrade) no matter how you choose to install it.

Using a VM (Parallels, VMware or VirtualBox) is the only way to avoid creating a partition for Windows. Within a VM, Windows exists on the same drive as the Mac OS as a large disk image.

With Parallels (at least the last time I used it), you can either set up a fixed image size, or use a dynamic image. The latter is the default. Preferable because you don't have to guess ahead of time how much space you think you'll need for Windows and the apps you'll install. It will grow or shrink to fit whatever you have installed in the Windows environment.

90 gigs isn't a lot for Windows. As an example, I originally had my Windows 7 space set to 50 GB. Since the OS and the very little else I had on it only took up 24 GB of that space, I figured that was plenty. Buzzzz! Wrong. Attempting to apply the free Windows 10 upgrade wasn't possible. Not enough room to both download the installer (plenty of room just for that), and room for the installer to create lots of temporary files to perform the upgrade. I had to bump my Windows partition up to 100 GB before I could proceed. So now Win 10 is installed, and after turning off a bunch of (to me) useless OS features that eat up GB of space, the entire Win 10 install and a couple small third party apps takes up 21 GB of space. 79 free to apply large OS updates with room to spare.

So your main problem at this point is you really need a bigger drive before even installing Windows. You have a 2015 Mac and only 120 GB storage space, so I assume this is an SSD drive. Not sure what Mac you have, so it may not be easy to upgrade the drive. I'd suggest a 500 GB drive. BUT!, since you'd be using a dynamic image, 90 GB of free space may work out fine, as long as you're not loading up the VM with huge apps (like games).

A caveat to installing Windows. You can use an upgrade copy as long as the Windows installer can see a legal previous version during the setup. Such as when I put Win 7 on my current Mac from an upgrade disk. I had my XP upgrade disk in an external DVD drive at the same time. That allowed Win 7 to self verify the existence of a previous legal version and install to an erased drive. Wouldn't have allowed it otherwise.

Oh, heck yeah. If it works to run the Windows apps you need, there's no reason to bother with the full OS.

Yeah, stick with what is working, especially if you have only a small amount of free disk space. You also need a good chunk of RAM if you want a virtual machine to run fairly well. I have Win 7 on a MacBook Pro, using VMware Fusion. It was OK for occasional use when I had 8GB of RAM installed. It was noticeably better when I went to 16 GB of RAM and was able to allocate more memory to the VM. For anyone with 4GB of RAM, I wouldn't recommend use of a Windows VM at all.