Air vs Pro, SSD vs HDD, is 128 enough? RAM? Retina?

Hi Friends,

It's been a few years since I owned a Mac, and now it's time for me to come back! So many questions...

I won't be using it for anything crazy, maybe a little bit of AutoCAD a couple of times a year, generally just the browsing, music, movies and whatnot. However, I would like it to last many years, this little Acer has lasted me 4. Prices in AUD.

I was thinking Air, 13 inch, 1.4 GHz, 4 GB Ram, 128 SSD, 1.35 kg, $1100 - plus 2 TB external storage.

Compare to Pro, 13 inch, 2.5 GHz, 4 GB Ram, 500 HDD, ??? kg, $1350 - probably still get 2 TB external for backup.

And to the Retina, 13 inch, 2.6 GHz, 8 GB Ram, 128 SSD, 1.57 kg, $1600 - plus backup as above.

Weight really doesn't seem to be an issue these days.

So, questions;

1. How much difference does SSD make? Are we talking the difference in boot up time of 20 s vs 1 min? Or, as I read on a review, producing 5 presentations instead of 2?

2. Is a slower processor with SSD better/worse than faster processor with HDD?

3. If upgrading any of the above specs, is money better spent on processor or RAM?

4. How much space are you left with after OS etc. with a 128 SSD? (my laptop is maxed out at 138 (strange number))

5 Is the Retina display going to save hundreds of dollars on optometrist and spectacle bills?

6. I'm happy to upgrade to future proof, which are the most important areas to spend??

Thanks to all for any input.

Curly.