Okay, it sounds like you don't have much experience with chainsaws, so maybe we should get you oriented before suggesting particular models.
It's not very helpful to mentally label saws in terms of the length of the bar. Any given saw powerhead can be attached to a variety of different size bars.
I guess I'd like an 18" saw - since those are the lengths I would be cutting.
It's the diameter of the log that determines how long the bar ought to be, but maybe that's what you meant. 18" is a perfectly reasonable length for general-purpose use.
Since any given saw motor can accept various length bars, the question becomes: what sort of powerhead will work best with an 18" bar?
For an 18" saw, you're going to want a motor with about 50-55cc displacement. A smaller motor can do it, but it will be underpowered and slow. A bigger motor might be fun, but is unnecessary.
From there we can ask, which 50-ish cc powerhead do you want? There are a bunch on the market.
The old brands most people associate with chainsaws they heard about when they were kids -- McCulloch, Homelite, Poulan, Remington -- have all been bought out and now sell cheap Chinese saws that are generally very poor quality. It's unlikely anyone here will advise you to buy any of these brands. Virtually all of the better saws sold in the US today come from either Stihl, Husqvarna, Dolmar/Makita, or Echo.
In general, each brand offers two or three different classes of saws.
At the lower end there are less-expensive saws targeted at homeowners and occasional users. They run just fine and with care can provide good service, but they're a little heavier, a little less powerful, and tend to be harder to work on when they need maintenance; a significant mechanical failure usually means a homeowner saw will end up on a scrap heap instead of being repaired.
At the upper end are professional saws, which are lighter, more powerful, and easier to maintain and repair. Pro-grade saws are usually worth fixing when they suffer a significant failure.
Some brands will also have a "farm" or "ranch" quality level that is in-between the homeowner and professional grades.
At $300 for 50cc, you're not getting a pro-grade saw unless you buy secondhand.