Rock - are you planning to completely inclose the shelving? If not, I dont think and type of insulation on the sides will have much if any increased affect on the temperatures. Depending on the lights that you use, they may be sufficient at keeping the plants warm along with any basic reflective material you care to line the sides with (plywood would work).

Again, with the small size of that unit, there will be very little air to keep warm. The smallest opening will make any attempts at insulating a waste in my opinion. I would rely on the lights itself to keep the plants warm in close proximity. If things get too cold, a small space heater turned on very low could keep things warm.

Yes, I was planning on completely enclosing it. My plan was to bolt the back and sides to the frame, and then having some type of door or something for the front. Maybe a sheet of mylar or the like? Even though I was originally opposed to it...
The overall dimensions are 18.5" x 49" x 7' (LxWxH), and all of the shelves are adjustable and can be cut in half if I need to.
I am completely new to this, but will be trying to work on the best options throughout the summer.
The light fixtures I plan on using are (2) sets of dual 48" T8, 6500K lights - 4 bulbs per shelf...
Depending on what I end up needing to put in there, I might have to use 2' lights on some shelves. I don't know! It's all about planning!
I was also thinking of painting all of the wood shelves white as well.

How cold do you project your garage to get? Im just wondering if the insulation or even siding is necessary. If so, i think that a grow tent might be an option for you. You can get one for around 110-120 on amazon.

They do a decent job of insulating, are completely lined with reflective Mylar and close up tight with zippers.

The price may seem a little high but I bought one for my garage and they are very functional in winter and summer. Just another idea..

Here is one that might work. Only thing i would be concerned about is the 49' width where the tent states its 48'. Im thinking it would fit but that would be your call.

Here is a link that might be useful: Tent on Amazon

It get's pretty fricken cold! It's un-insulated, cracked foundation/floor and no ceiling - I use it for storage...
That grow tent looks awesome! I would definitely be willing to go with something like that.
The width is actually 48.5"...
Just out of curiosity, I will search the forum, but does anyone have good/bad things to say about the different brands available?
Hibiscus, hoyas, succulents are pretty much my main focus... maybe a few others that get added through the summer...

I bought a grow tent made my Avidity and could not be happier. Very good construction!

This is not the one I bought, but comparable. This would be great for you if it was just a bit taller!

Really good tent for a very reasonable price.

Here is a link that might be useful: Aviditi Grow Tent