SSD in iMac 21.5" Mid 2011
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.

Good luck!