Unfortunately, you can't look at it simply as an '07 for $5,300. You should expect to have to put money into it starting soon after you buy it. If not the two most expensive components-the engine and trans, there's all the little things that can easily add up to the same expense as them. It's not just the parts and labor, but also the hassle of always having it off the road to have something done to it. That'll suck the enjoyment out of it. You may end up putting a few grand into it and get it solid and reliable (well, seemingly reliable because at this point, there are no guarantees), but then you'll have $8,000+ invested into a ~350K-mile '07 that you may want to offload because you're tired of fixing it. You sure as hell aren't gonna get that money back, not to mention even less your initial $5,300 purchase cost.
Furthermore and personally, I'd feel better about buying a 323K-mile GMT800 than a 323K-mile GMT900. For $5,300, you can get a GMT800 with at least 100K miles fewer miles.