These are absolutely great fun watches. I bought loads of these when I first got into watches 15 or so years ago (back then £50-£100 bought you a decent one!).
You're correct on sizes - 28mm and 34mm (36mm with crown) were the order of the day.
Buying - auction (ebay seems the best route).
What to look for - decent bezel and strap. Aftermarket and NOS parts can still be had for most models though.
These are all steel/plastic models so really not the most durable, and bearing in mind the first gen are 37 years old you'll pay a premium for good ones.
I've long lusted after the bright orange/grey model but never found a good example. I had the black and blue one though, loved that watch!
Here is a fantastic link to a history of the early F1 models; https://www.google.co.uk/url?sa=i&ur...AAAAAdAAAAABAK