I mean if you dont care about the cost, then apple computers can be great. If you only do simple things like photo editing, video editing, browsing etc. But as you said you want it for IM and then I must say I dont think that will work out that great. Their computers are fancy, compact, great design. But for that cost, and such limiting support for software I would never buy anything like that. If I didnt care about money at all I could think about buying an Imac for my children since they are pretty easy to understand for beginners.
What kind of computer you should get is very hard to say because there are so many diffrent configurations, you have to tell more about what you want to use it for

If you wont play games or do alot of photo editing there isnt any reason to get a high end graphics card which is usually the most expensive part in a computer nowadays.
If you really care about price and want the best thing for you bucks like me, I really suggest to buy many things used. And to build your own computer

I right now have a top of the line computer which would cost about $2,000 new today. And I have spent maybe $1,000 for the parts. Maybe a little bit more.
If you choose to buy a windows PC, and to buy it not used I suggest you wait some weeks untill Intel Skylake processors are released this summer

Also you HAVE to buy a SSD as hard drive, or buy a computer with an SSD as a hard drive. They are really cheap nowadays and trust me you wont regret it. The computer feels 10-50 times faster, even with low end CPU/GPU/RAM.
Good luck!