Bison and cattle are quite a bit different in the habits and foraging abilities/preferences. If managed correctly, each could help the vegetation management. There evidence in UT that free ranging bison are doing more harm than the cattle that previously grazed there at one location. In another locale, the different foraging habits of bison have changed the vegetation which has greatly reduced the amount of elk use in some areas. So, just replacing cattle with bison will not always make things "better".
One big difference I see with the two of them on public land (regardless of state or federal) is that generally the cattle are allowed through a permitting process. This becomes important, IMO, because that means the cattle may or may not be allowed on the property year round. So, on a very dry year or to do vegetation management the cattle can be removed for the year or a period of the permitted grazing season. If this property is to be used as a bison 'refuge' where do they go on dry years or years with short feed or if an area needs rested due to vegetation treatments? Likewise, 27.5K acres won't feed all that many bison year round. Since I am totally unfamiliar with lands being discussed, the numbers I am throwing out are completely pulled out of my arse. Say the area is producing forage at 1 acre/AUM, which I wouldn't doubt is possibly and probably conservative. That means that total forage being produced is 27,500 AUMs worth. Lots of literature suggest that at a minimum 50% of forage needs to be left for recruitment and watershed health, so to be conservative lets assume that 65% of forage will not be grazed. That leaves 9625AUMs for all animal consumption. In order to be conservative, lets say that only 30% of the allocatable forage is allocated to bison; 9625X 30% = 2887AUMs. Bison use approximately 1.5 AUM of forage. So, 1.5 X 12 months is 18. That means in this hypothetical scenario the piece of ground could support 160 head of bison. This assumes that the forage is evenly distributed and every acre is available/suitable for bison grazing every day of the year.
Those are some concerns that I would think need to be raised. However, I agree with Buzz the most important thing is to get the land purchased THEN work on a management plan.