In the first link Occupant listed, it states that you should ream only .002-.003 of material. That's good practice but that's not always possible. I like to keep a drilled hole about .002-.005 smaller up to an 1/8" diameter hole. Above that I like to leave approximately .010 of material to ream. That is not always possible to do that either. For instance, in your standard drill sizes, the next size smaller than .250 is .234. That is a .016 difference. That's pushing the limit for material left for reaming. You would have to use a Letter Drill "D" which is .246. I have reamed .016 of material many times, but it was in aluminum. Then again, the depth of the hole, the type of material, whether your drill was dull or sharp all come into play. Once you get into the habit of reaming your holes on an everyday basis, you will find what works and what doesn't.
When you purchase reamers, you want to be sure to purchase a couple extras if money allows it. 3/16", 1/4", 5/16". Those three sizes I have found, are used quite extensively in various things you may be working on. As far as Metric sizes, which I have never had the opportunity to work on metric projects, I would imagine that there may be two or three metric sizes that are used more than the other metric sizes in a lot of applications. So a good set of Number Reamers and a set of Fractional Reamers should get you started. You won't need a set of Metric drills if you have a set of Metric reamers. Possibly later on you could pick up a set of Letter Reamers. I have found in my work and all of the different things I worked on over the years, Letter sizes didn't crop up to much.
If you ever have the opportunity to work on pulley's that have keyways, or you are going to make pulleys with keyways, try to remember that you should always ream a keyed pulley with a spiral flute reamer. This keeps the reamer straight in the bore. A straight flute reamer can catch a keyway and either snap your reamer or make the hole out of round.
Enough of that though.

I like the idea of the coolant through the toolholder for the lathe. I don't think our machines at work have those. Usually everything was flooded with coolant. If we did have them, I never seen it. But I can't imagine that the sales reps wouldn't have mentioned it at some point in all the years they were coming in. Myself, I like the idea.