You could try something like Linux Mate. and use a Virtual Machine for Windows programming.If you don't want to, that's cool.
You can find windows ISO images and serial number on the internet. Yes, there is a risk, but you are installing Win 7, a 13 year old OS, so putting it on the internet would be a mistake.
There are people selling the serial keys on eBay, but again you don't know how many times they have sold the same license.
I am not sure if you can still buy a license of Win 7 from M$, but that would be the safest way, but most likely the most exspensive.
Yes, the Win 7 disk should boot, so then I suggest that you remove the partitions and install win 7.
The OEM software is for brand name PC's like Dell, etc so it will not install on a non-dell machine.
A standard version of Windows will install on most hardware, but the drivers will be required to make things nicer.