How much extra space do you need during the process of executing a migration from iphoto to photos?

Where is your library located? On your system drive or on an external drive? And is your Aperture library managed or referenced? A referenced library will need about the same amount of storage as the aperture library is using. A managed library less. But the amount will depend on how large your original image files are.

The migrated library will be created on the same drive as the original library, otherwise hard links will not work.

To estimate the amount of storage needed ctrl-click your Aperture library, select "Show Package Contents", and check the size of the folders inside. Only the images in the "Masters" folder will be shared by hard links. Subtract the size of the "Masters" folder from the total size of the Aperture library.

I only have 25gb left on my hard drive.

That will not suffice at all, because you must not let the free storage drop below much less than 20GB, or you may experience a poor performance if the system does not have enough free working space.

Move your Aperture library to an external drive and migrate it on the external drive. Later you can swap the Aperture library on the internal drive with the new Photos library.

Make sure the external drive you want to use is formatted MacOS Extended (Journaled) as described here:

Format external drives to Mac OS Extended before using with Aperture

The migration will take place on the same drive as the library exist on that you are migrating from. If the source library is on an external drive, then the new Photos library will be created on that same drive.

So, the additional space will have to be on the same drive as the iPhoto library.

How much space to migrate? I don't know for sure, but I assume you will need as much free space as the size of the library you are migrating for the process to take place. The end result will not consume but a little more space for the new database files. Only the photos are linked together.

Aperture libraries and iPhoto Libraries are the same, there will be no difference during the migration.

It will not suffice to free space just for the migration. You will need additional space afterwards. The iPhoto Library and the Photos library will share the original Masters files, but the other library files will not be shared - previews, thumbnails, faces, database files. your storage used may increase by a fourth of the current library size. It is hard to estimate how much this will be. It depends the size of your Master files. You should not let the free storage drop below the 25GB you have now.

Instead of moving your iTunes library back and forth, you could move the iPhoto library to an external drive, migrate it there to Photos, then copy the migrated library to your internal disk. The migrated library will have about the same size as your iPhoto Library now.

So after I do as you say, when all is said and done after copying the migrated photos library back to the internal drive, will I have both iPhoto and Photos to use on my iMac without the plugged in external HD where the migration took place?

No, if you follow that procedure, you will have to plug in the external drive to access the iPhoto Library. when you migrate the libraries on an external drive, they can share the files only while both libraries are on the external drive.

It is unlikely, that both the iphoto Library and the Photos library will fit on your internal drive, with only 25GB left. It would be a tight fit, and nearly no storage left for the operating system as working storage. The mac would be terribly slow and you would risk crashes.

So one of your libraries needs to stay on the external drive, your iTunes library, the iPhoto library, or the new Photos library.

It is not easy to take the new iMacs apart and to exchange the internal hard drive. Have a look at this Macworld guide:

http://www.macworld.com/article/1159956/imacharddriveupgrade.html

(wait for the advertisements to finish, the page comes up a bit later)

But since you are having a desktop mac, that you do not carry around, an external drive is not a big hardship. The new 1TB drives are so tiny, you will barely notice them sitting on your desk in addition to your backup drive.