As long as you have a genuine product key, you should be able to download the ISO for Windows 7 from Microsoft. Here is the link:
MicroSoft ISO Download
Just enter your key and it will direct you to the correct ISO. Once you've downloaded it, burn it to a DVD and you're good.
As for migrating, unless Samsungs migration software is different, it will take an image of your present HDD. So this image will require at least as much space as is used, so in your case 1TB. Since the destination is 500GB, it wouldn't work.
The best option is to leave your HDD unplugged while installing Windows on the SSD. Once you've installed Windows, drivers, and software you want on the SSD, reconnect the HDD. Copy off the data that you need from the HDD to an external drive. Then format the drive and put what data you want back on it. The upside to this approach is you haven't touched your working OS installation until after you have the new install complete on the SSD. So if anything should go wrong, you aren't without a computer while you sort it out.
My suggestion is to put your user files (Documents, Pictures, Music, Videos, etc) on the HDD. It's pretty simple to move the location from the default C:\Users\Username\ to wherever you want it.
If you are a gamer, I would install STEAM on the SSD, but then select the HDD as the location to put the games. This can be done within STEAM's settings. Same for Origin. Then any non STEAM / Origin games that have their own installers, direct the install directory to the HDD. For instance I use a directory like this D:\Games\ to install all my non STEAM / Origin games in.