veganbikes

You cannot add a freehub to fixed gear hub. In some cases you can add a freewheel but a freehub requires a cassette hub.

The freewheel does matter as does any gearing you add. The lower cost stuff tends to be less durable and less round and other things like that. With a single speed or fixed gear buying higher quality stuff can be a net good for the bike. However if you are just trying gearing to see if it is right buying something lower cost is sensible for a short bit to test.

In terms of different handlebars it is generally always possible but it can change a lot of things and without knowledge of the bike and your current fit on the bike it is tough to say. However typically you will need new brake levers and need to make sure the bars will fit the stem or you get the correct stem which is most important. Don't just buy stuff for looks either make sure you are comfortable on it. Track bars aren't always the most comfortable but practical on a track setting where everyone is going the same direction with the same goal in mind on the road you may want a different bar.