I am just getting into starting seeds. I have not ever had a place to do it so always end up buying already started plants from a greenhouse or Home Depot. Not everything I want to grow is available when I am looking for it if at all in my area. That is the motivation for a greenhouse. Learning a ton about greenhouse growing and trying to learn from the mistakes others have experienced so as not to repeat their fails.
I have seen the plug trays. I am also looking at perhaps getting a seed starting area set up in the laundry room. I have the space but only one electric outlet in that room. Good thing I have an friend that does commercial electric.
I am agreeing that trying to start the tiny seeds may be hit or miss in a greenhouse without heat. I also agree it could be costly to heat it. Or worse yet burn the structure to the ground.
I am looking at some lighting options as the laundry room although large has only one TINY window.
This is kind of expensive in my under educated opinion.
Lots of research needed before I commit to a grow light set up.
Reading and learning a ton. I hope I retain some of it.