The M50s will require amplification of some sort, as will any larger speaker that is not self powered. There are self-powered speakers out there but I'm not familiar with them enough to recommend a brand. I have read a few good reviews for self powered models but it seems that most high end speakers rely on outside amplification. Also be aware it means that you need an electrical outlet wherever they are located.
You don't need a receiver if you are only going to run the iMAC to the speakers. A receiver will simplify things later on if you start to add other devices but if it's strictly iMac to speakers only, then all you need is a stereo amp in the middle to give the speakers clean wattage.
Some stereo amps have a subwoofer out module but some do not. Even if it does not, you can still use a sub but just make sure it has connectors to tap into the speaker wire heading to one of your speakers, as opposed to just an RCA type plug only that wants to connect to the sub out of a receiver. They both do very different things.
Even the above can be worked around but a sub that is designed to run off a speaker feed as well as a sub-out, will be much more likely to have a built in crossover to ensure it plays just the bass frequencies below a desired level and not the full range of sounds going to the speaker. Normally this is handled in the receiver or amp with a proper sub-out.
Depending on the speaker you buy, you don't necessarily "need" a sub. The M50s or other similarly sized/quality speakers will sound very good on their own. You might not need to buy a sub right away if the budget is tight but I'd suggest getting an amp or reciever that supports it so you can try it out down the road. A good sub will enhance and smooth out the bass sounds of even large floor standing speakers like the M80s. Do you need it "no" but would you like it "Yes."
The best speaker depends on some variables. Can you please further explain your room size? Your musical interests? Do you prefer your music to have exaggerating, Kick-Azz, chest rattling bass or do you prefer your sound to be as close to accurate to the original recording as possible? How loud do you listen? Most here don't encourage unsafe loudness but being capable of dishing it out is a deal breaker for some.
Wow, that very detailed. My room size is approx. 2000 cubic feet.
Say I use my iMac for occasional game playing, video watching and music playing.
What would be most advisable? 2.1, 5.1 or 7.1
Game playing :25%
It is ok if it is in wall speakers cause my house is being constructed, it can be built to fit.