Well, I prefer Fantasy or modern minis, but I sometimes buy other minis if I like them.
If I just need 1-2 minis and can get them in one of the shops around here (mostly GW, Rackham, Reaper) , I'll buy them there, if I want Stuff from other companys I usually wait a bit any buy a load at an online shop.
It doesn't matter to me if they have nice paintjops or something, I don't mind having only seen the green or bare metall. So I won't have any illusions about how it will look in the end. (I'll never be able to paint like the Rackham artists.)
I usually buy 28mm, but I don't mind if they're a bit smaller or bigger. I have some 52mm minis, but I have problems painting them, so I only buy them if they really are gorgeous.
I don't mind the bases, when I like the mini, though I hate Reaper bases (who don't). I substitute all other bases mostly with the Dark Age ones, or make fitting efaplast- bases
Packing I don't care about, as long as I can see the miniature. Reapers packing works good enough, though they're allways bending the wepons. And I prefer if it's whole in the package, I don't like multipart minis much.
I prefer single minis, but if I really want one mini in a box, I might still get it.
I just buy what I like, though there are some lines I like much stuff from, like Hasslefree. I tried painting myself an Confrontation armee, but it didn't work. I just can't paint what I don't like, and all in the same colours and stuff.
So, why do I like to paint minis: I'm not even sure about that. I guess it's for the beauty of it. Just to paint something so that it looks good.