I would say that trying to use 1 fermenter might be a little bit aggressive. You could double size your fermenter but think about how you plan to use them. I have a double sized fermenter but really wish I had just stuck with the same size as my mash cooker. In the winter when the tap temp is cool I can get to mashes done in a day to fill the fermenter, but in the summer the crash cooling takes a little long and my day becomes too long for a double mash. If you run two shifts this is irrelevant. You can add the next day as your mash hits vorlauf, but make sure you cooling jacket goes low enough to have enough surface area to keep you mash cool between day one and 2. You can drop the temp of your fermentation lower to make sure you don't get too hot before the next mash is added, but again, in the summer this can get painful. This is assuming no chiller. At 300 gallons you might be able to get by with macro bins or something of that nature and save money there. Not sure how your temp control works in the size range. When I started I was using 55 gallon drums which never got too hot, but when I jumped in size I was amazed how the mash would heat up. Mash cookers are not real expensive so you could get a bigger one, but realize you are going to need a pretty big boiler to run a 1200 gallon mash cooker and still (probably at least 40 bhp).
Good luck.