I am planning on building a large enclosed (Cartesian XY-Head) 3d printer. I want to keep the inital build time minimal and it is very likely I would need to build the ~50x50cm heatbed from scratch, because it doesn't exist in ebay. The enclosure itself is not separately heated, but depends on waste heat from the printing process. The enclosure will eventually be vented outdoors with a 12V CPU vent via ducting (air flow unknown).
Do I even need a heated print bed, when I am planning to only print PLA?
The answer is "No" you don't need a heated bed for PLA but it does make the base layer a little easier to lay down and also helps with print removal post print.
PLA is a very easy filament to work with and the majority of PLA printers don't come with heated beds and suggest blue tape and/or elmers glue.
You may find that if you are purchasing very cheap/poorly extruded PLA, that things don't stick as well and a heated bed can alleviate some of these issues.