A heated bed is not necessary for PLA. But, pretty much everything else will need a heated bed. The main purpose for a heated bed is to keep the part in place while it's printing. There are many other methods for doing this including blue tape or a slurry application (I use a slurry made from Elmer's glue and water). This works great with PLA as PLA printing is relatively low temperature.
For other materials, you want to maintain a high temperature at the base. Otherwise, it will cool and shrink. This usually results in the part lifting off the bed. At a minimum, this will prevent you from getting a flat surface on the printed side. Or, the part will come completely off the bed. The printer won't realize this and will keep printing. Plastic will eventually collect around the hot end, encasing it in plastic. For some hot ends like the Ubis and anUbis, this will ruin them as the plastic will make its way under the insulating sleeve.
Ask me how I know ;)
Worst case, I've heard of this causing fires.
So yeah, a heated bed is pretty useful. To get that build area, you can combine two heated beds. You will probably want to connect them to their own power source though.
For that build volume, your biggest nemesis will be shrinkage in the part. You will want to print in an enclosed area where the temps are kept high and there is no airflow.