Upload & Sell: On
I was in a similar situation as you last year. I ended up starting with an old Sigma 120-300 non-DG and sold that to buy a Nikon 300 2.8 AF-S II. The old Sigma had slower AF and was not as sharp as I would have liked, but you could get by with it. With TCs though it was not sharp enough for my uses. I bought the Nikon AF-S II because it has super fast AF and its razor sharp, and still very sharp with the newer Nikon TCs. In addition, the AF-I and AF-S I models have issues with availability of repair parts for the AF motor, whereas the AF-S II seems to have fewer issues now.
The AF-I runs around $1800-2100, AF-S I around $2500, AF-S II $2800-3000, Sigma 120-300 OS $2000 (US Dollars). If you consider this a purchase to last 5 or 10 years or more, then I would suggest spending a little extra and getting what I consider as the best of the bunch, the Nikon AF-S II. I think the difference in cost over the others will pay off in the long run in terms of number of keepers and longevity of the lens. Plus the resale value will be higher as well for these same reasons.