Primer?

Yes. Priming will help you see any imperfections and give the paint a surface it can cling to.A light coat is best.
Tamiya and most hobby paint companies have small aerosol cans of primer. I use a cheap Dollar Store Brand. You get twice the primer for about a $1. Light grey or white seem to work the best

Tamiya has something called a "Flat Base". What exactly is that?

Just a guess. I believe it is used to dull a gloss color and/or dull a flat color even more

I myself swear by primer for any material that is to be brush painted (of which figures is the most obvious).

I prefer to buy the expensive stuff ($10+ Canadian per can, one can will prime 50-100 figures easily if you do more than one at a time). I used to use auto spray primer and swore by it but a few years ago bought the expensive stuff and will never go back. My favorite is by Citadel (Games Workshop), it's a popular brand amongst fantasy war gamers. I think the Games Workshop primer (called 'Skull White') is made by Krylon so their primer would most likely be the same. I find the primers I mentioned to have a smother finish but.

Something Charles didn't mention... A good smooth primer coat helps hide brush strokes.

Hope this helps.

Tamiya Flat Base is for mixing in with gloss paints to make them go flat. It is not a paint in it's own right.