I don't think we will be much help -- range and coverage depends on a lot of environmental factors, including the nature of the foliage / trees in your area, as well as how the AP is mounted and what kind of devices you intend on covering (phones have much lower range than laptops). And in your case, which external antennas you decide to pair with your T300E.
If you are trying to select the right AP for a deployment, it might be better to work with a supplier and get some actual units for doing a site survey.
River is not a problem - trees are. As far as you have line of site, you will be fine up to some 100-150 m from AP. But recommended way would be to get AP and make site survey, if not with professional tools, tad at least using some free tools (Inssider for example) on laptop, HeatMapper or Wi-Fi analyser on any Android device. To get an idea of possible coverage, you even can use any AP, T300 just will provider a bit bigger distance and much more speed everywhere, but even with worst SOHO AP you'll get some idea about coverage area -- as frequency and modulation are same, propagation is similar. Of cause, it apply only if you don't plan to use directional antennas - than you obviously need to survey with similar antenna. Actually, for me idea to use external antennas with Ruckus is the last resort, and viable only in very specific cases, because this way you loose the main benefits of Ruckus equipment -- Beamflex. Also, external antennas are potential source of problems because connections waterproofing isn't so reliable. Mostly when you need directional antennas you just use T301S and N models with integrated 30x120 degrees and 30x30 degrees antennas, which work absolutely brilliant. We regularly use Ruckus AP with 30x30 degree antennas for public events on sea beach (musical festivals) and just mount some T301S APs on the one pole in the area, target them in proper directions and cover big areas with many hundreds of users. Anyway, if area you want to cover isn't compact and around some pole, you need directioanl antennas, and you would prefer to use T301S or N -- with T301N directional antenna you can efficiently cover long area about 30x 500m or even more, if there are no obstacles. Of cause, T301E with similar sector antennas will work good enough, but T301S for example, would work better - you have no additional signal losses on cables to antenna, and you have a few additional db because of Beamflex+ ... If you don't have experience and equipment yourself, it would be a good idea to consult some company with experience with outdoor projects, they would have equipment and experience needed.