$400-500 is a good range to be in, that opens up a lot more options for you, and gets you a much better saw.
Heres the thing, I always encourage people to buy a saw one size larger than what they think they need. I learned this lesson the hard way myself. I had been content with my Husky 350 then took the plunge and bought what I assumed would be the largest saw I would ever need an MS390 with a 20 and 25" bar. The first tree I went to take down to sell firewood was too large for the 25" bar to reach through and there wasn't really a safe way for me to fell the stump. I sold it and bought a hot rodded 046 kept the 25" and bought a 32".
Right now you have a bunch of deadwood to clean up and I gather it's on the small side, but this isn't a fire place, this is an OWB, it takes a lot to feed these beasts. What are you going to have to cut up for next year, or the year after that? Is that 18" going to keep cutting what you can come across to feed it? Remember you are talking CORDS of wood, not a cord or two worth of cutting.
I don't know, maybe all you have is a bunch of 14" tree's and they never get bigger than that. But I know that I have used my 32" bar a lot more than I like to, enough that I came across a deal and bought another one to have a spare!
Think about your area, think about the trees, look around surrounding areas you may have to cut on to get wood, how big are the trees? If you cheap out on a saw now and find out i's not large enough o powerful enough to keep up how frustrated are you going to be when you have to buy another one to replace it?
I really am just trying to help you out here, this is the reason I don't have an OWB myself, it's a commitment!