You don't have to worry about it. Simply hook up the two machines together by USB cord and let Migration Assistant handle everything. It will not bring over your system or your Apple default applications, or any third-party apps that don't run under Yosemite, so that's likely to be a savings of a few GB right there. You can also tell it not to bring over certain classes of data, like extra users or any of the applications (it's not very specific, however).
As for your "programs I don't have the disks for," they'll transfer just fine -- but depending on their authorization schemes, they may not run. You'll need to contact the companies you bought them from to get them working again.
So what you'll end up with is a very full (but not quite as full) new MacBook, from where you can then move stuff like the iPhoto or video libraries onto an external drive to free up way more space. A good rule of thumb is to leave a couple dozen GBs around at all times for free space, in your case that would be 25GB or so (more is generally better), so tend to that ASAP after the migration has completed.
It's important to do the migration the first time you boot up the new MacBook, right when it prompts you to during the initial setup process. Trying to do it later is a big headache. In the meantime, do a spring cleaning on the old one, pare down as much as you can, and maybe move some space hogs (like the photo library) onto a USB thumb drive or something where you can add those back in later as needed.