Welcome,
To answer some of your questions:
1. When looking at side burners, Look for a heavy gauge of steel. The heavier the metal the better it will hold temps. Also look at the gaps in the steel around the fire box doors, the better the fit the better it will hold temps. And you are correct, Side burners do tend to be hotter closer to the fire box. The main problem with inexpensive side burners is the opening from the fire box into the cooking chamber tend to be to large. You will also need to make a baffle to direct the heat down and into to cooking chamber. Inexpensive side burners usually don't have a good fire grates in the fire box or come with charcoal baskets. Wood grates or charcoal baskets are easy to make.
2. "Heat Plates" I think you mean "tunning plates," they are plates of metal that sit below the cooking grate and are spaced ( tunned ) to allow the heat to travel evenly across the cooking surface eliminating hot spots.
Having said all this there are a lot of good inexpensive side burners out there. I have a side fire box
Chargriller Pro which I love. here are the mods I have made. It will give you an idea of how to make a good smoker even better:
I have lined the cooking area with fire brick to help even out the temps and to help hold the temps. When the bricks heat up,( It does take a little awhile) they help hold my temps and even out temp spike when I add wood. The
Chargriller all so comes with a Charcoal Grate that is to be used in the main cooking chamber so it can be used as a grill, I took the Grate and turned it upside down and it acts as a baffle to direct the heat down and away from the fire box and into the cooking chamber.
I hope this helps you in your search.
Shop around and look for your best deal, also I am sure the nice people here will give you advice when you decide on a particular model.
Good Que'n
Ron