Hey there! This is my first post, but I've been reading the site and testing out different configurations on my mini 9 for the past couple weeks, so hopefully I can answer all your questions.
1. It doesn't matter which OS you buy the mini with. The install process is not exactly the same, but you will have to format the ssd
drive either way, so whatever OS you buy the machine with will be wiped out anyway.
2. You can install OSX on the mini 9 base model, except that you need at least an 8gig ssd
drive, not the 4 gig. Even then, most users seem to agree that the 8 gig makes the OSX installation difficult, so you really should get at least a 16 gig ssd
drive. You don't need to buy that upgrade from dell, there are after market seller who can get you the bigger drives much cheaper than from dell. Check out mydigitaldiscount.com or ewiz.com. (Make sure you are looking at the Half Mini PCI-E Solid State Drive).
Here are my 2 recommendations: get a refurb unit from dell outlet now while they are having their 15% off spring clearance sale. And get a machine with a camera, because from everything I have read, it is nearly impossible to install one on your own later.
I hope that helps.