While an iMac is not the least expensive computer that you can buy for AE they have with out a doubt the best customer service in the world with the most forgiving warranty. They also hold their value very well. A first generation intel iMac I purchased when they were introduced at MacWorld in January 2006 was sold to a client 5 years ago and is still being used every day with no problems. It won't run CC because it doesn't meet minimum system requirements any more but there are very few 2006 generation PC's that are still being used every day.
Can you get a machine that will render faster? Yes. Will it work for a designer - sure, lots of designers work on iMac's and MacBook Pros.
.....you should read more on what exact hardware is needed for adequate performance with AE. Much depends on the video codecs in use and the and the nature of what you will export. AE uses ALL the memory you can throw at it AND it has a "global cache" feature which allows for the "storing" of rendered frames to improve performance....that cache should be on a high speed drive like an SSD.
Windows machines currently are CHEAPER and offer more flexibility in constructing a machine for AE. The Haswell E series CPUs on an X99 motherboard is a good start......an overclocked 5960X to 4.5 Ghz provides EIGHT CORES, ( 16 "threads" ) of processing power !! The X99 board uses the new "quad channel " memory with higher speed RAM memory. 64GB is a good start. That motherboard also has 10 SATA III PORTS for connecting many SSDs AND has a direct PCI connection for the new Samsung SM951 PCI SSD which runs at close to 2 GB/sec. read and write.
If you are SERIOUS, you need a capable machine.......the Apple products are costly and may not offer the best hardware for AE performance. Read more first, before choosing.