DSLR have about as much shutter lag as your N90/F90, so nothing you feel. i have the canon 9000f as well and tried scanning with it. BW seems to be quite ok but i struggle very much with colour.... looking forward to hear how that goes on your side.
tank processing of BW film is indeed easy, however not sure how easy it is to get BW film in thailand, here in Singapore only a few shops are still stocking it and for a medium format roll it is 3-5$ or 75-125bht. so have fun with your F90, collect some money and then get a 2nd had D300 (by that time the D400 will be out and D300 will be a bargain). btw have you ever checked out
www.keh.com great site to buy second hand equipment