In 320 hours the tractor is probably mechanically sound. Because it was outside check the hoses and tires for cracking and wear. Mine was perhaps outside and the issues on mine so far are Check the folding ROPS and the seat slide. Mine must have been left outside and the ROPS is rust welded into a none folding unit no matter how much penetrant I put on it so far. and for what ever reason my seat adjustment is stuck and I can't break that free either. The seat on mine was toast to and I paid a couple hundred for a new one I think. Also the electronics like the RPM seems a little buggy maybe due to outside storage.
If the brush hog is a 6 footer landpride or other "decent" brand and the back blade is like a nice thick heavy heavy duty one; that is worth 1500 maybe. If they are King cutter or TSC, those are plenty serviceable too, just not worth as much thrown into the deal. A heavy blade makes all the difference in the world. I have a 6 foot deer brush hog and 5 foot TSC and they both do a good job I just feel worse when I back the deere into something it can't bend over and have to buy new deere bumper parts.
The low hours makes it worth more. The storage outside makes it worth a little less, in my opinion, because of all the rusted fasteners you will probably have to deal with (at least in Michigan they end up that way)
If you pay 13,500 for the package you got a better deal than me and I think it is worth that
17 sounds high unless it isn't showing any outside wear and tear.