Looking to get into a drop bar road bike, single speed (non fixie) to have a dedicated Silver Comet bike. Have many questions but don't know how to ask them due to ignorance about this topic. Is it easier to convert/build a road bike frame, along with a chain tensioner, or go with a track/fixie type and have to worry about rear wheel alignment?

Personally, I wouldn't want a tensioner. The whole idea of SS is simplicity and an efficient drive train. Aligning the rear wheel is very easy.

Are most SS bikes easy to change chainrings or cogs or do some come with a components that don't allow for that?

Easy. Except for some of the cheapest bikes which come with cranks that do not allow chainring swaps. Rear cogs and freewheels are pretty much standard and can be swapped as needed.

I've been trying different setups for a few years. If converting a road bike with a freehub I suppose an 8-10 speed freehub is preferable to an 11 speed freehub, though I don't know that from experience. You could use Shimano, Origin8, and Wheels Manufacturing for cogs and spacers. The threaded freewheel hubs are generally significantly longer in between flanges from what I've noticed. But, you said you weren't interested in fixies and you don't need to get into threaded hubs. If you are interested in track style riding then I think a threaded hub may be preferable.

You may need a vice, specific vice wrench, chain whip, lockring spanner, and cassette/freewheel removal tools, though maybe not all of them. I'm currently deciding between a Pedro's vice wrench and a chain whip style lockring tool, and would go for the vice tool if I was working.

Trying to recall whether a rear hub ever takes a lockring spanner but not certain.