5 yo is young, I bet you he'll be changing quite a bit for the next 2-3 years or so. I also agree with UA and Faye that most arabs I've seen have very short backs and there is a danger that 18" may not fit him (my fitter had similar concerns with my short-back paint, but thankfully my 18" is still OK on her).
As for brands for $1000 you have lots of choices. I myself have Thornhill Vienna, but it's designed towards wide horses, so I don't think it'll be a good choice. M Toulouse is more narrow, so something to look into and it's around $1000 new. As for used ones you can look at Passier, Crosby, County, Albion. I didn't like KN (felt very hard) as well as Stubben (same issue), so while those are saddles to last they are not on my list.