Will using a ventilation system with an heat exchanger give me the same quality air indoors as outdoors

Start by removing the trim from around the window, this will give you a good look at the problem. Once you have the trim removed you'll be looking at something similar to this...

You may notice gaps between the window frame and the wood frame or between the wood frame and the wall, these are the areas you'll want to fill to stop the draft.

Get a can spray foam (like GreatStuff they make some specifically for this purpose, which is "low pressure" so it will not bow the window frame), fill any gaps (be careful not to use too much, this stuff can expand quite a bit). Once the foam has hardened (about an hour), use a sharp knife to trim the excess foam so it's flush with the window frame. Reinstall the window trim, and your done.

Is it completely necessary? No I don't think so. As a comment pointed out, you are creating negative pressure. This is good for sealing environments (ie: keep vapors from existing the room in a lab), or even help prevent fire from spreading via halls.

However, you will notice some negative effects to. For one, since the air will be pulled in through any cracks under doors and windows, if a door is not latched, it will likely be forcefully opened. Depending on the pressure, this could potentially hurt a young child who is not expecting the door to be pulled open by the pressure when they turn the handle. Having a second fan push air into the room aleviates this by balancing the pressure. It also reduces the strain on the extraction fan which may help it last longer.

Another benefit is that if one fan fails, you still have a second either pushing cool air in or sucking warm air out; it will function with reduced capacity, but it will certainly be better than no fan at all!

My guess is that you will need two smaller fans instead of one large fan, which might also simplify electrical requirements, installation and maintenance.