I'm not sure if I should buy an SSD for my iMac 21.5" Mid 2011 alongside the internal 500GB hard drive. First Question: Should I replace the CD drive with an SSD or use an external (Thunderbolt) one? Is that hard, and is it easy to keep all the applications and system data on the SSD, while storing media on the HD? And should I buy an SSD now or wait for maybe a year until cheaper and better SSDs come out? Thanks!
In the Mid 2011 iMacs (both 21.5" and 27") Apple left enough room for an extra SSD. So if you want to put an SSD inside your iMac you can do that without replacing the DVD drive and/or original hdd. If you're going for an external one you should indeed opt for a SSD with a Thunderbolt connection, otherwise you kinda loose the speed advantage of a SSD.
If you want to separate the OS and application data from your media files, that's entirely possible and there's more than one way to achieve that. Read this and decide for yourself wether it's easy:
SSDs will only get better and cheaper over time so it can make sense to wait, but since they'll keep getting better and cheaper you could also wait for the rest of your life but SSDs are becoming mainstream now so it is safe to make the jump.
Remember that if you install a SSD in your iMac yourself you will loose your warranty so it may be worth to let an Apple Authorized Service Provider do the upgrade for you. If you insist on doing it yourself you might find the following link useful:
I can tell you that unless you have extensive experience working on iMacs and have all the right tools, it's a huge pain. Even for Apple technicians that have all the right tools and training, it's not the easiest machine to work on.
I would recommend using an external solution if you can. However, using an external SSD to boot from isn't the most ideal solution since it's not going through the internal SATA connection. Configuring it this way will cause you to erase your internal iMac hard drive, which isn't the best idea. My recommendation would be to take your iMac to an Apple Authorized Service Provider and have them perform the SSD addition since they'll have the right tools and training, plus they'll be liable if they mess it up. As for the waiting to purchase the SSD, we are unable to make shopping recommendations on Ask Different.
Certified Apple repair shops can install an SSD in your iMac without voiding your warranty or Apple Care, its their job.