An F100, 35mm AF-D f2 and your current flash would be well within your budget. The 35mm f2 is a nice sharp lens. Reasonably sharp wide open but becoming very sharp a stop or two down. The R5 and a Leica lens suggested above might sound tempting but, from what I've read, the R5 doesn't have the best reputation for reliability. Certainly the F3 should be better in that respect and the F100 as well, I'd imagine. Plus, if you wanted to add a second lens, a good Nikkor would likely be cheaper than another Leica lens.