If you only manually focus with an F4, then the AA batteries last quite long time. The autofocus and motordrive are what eat up batteries, though even then you can do many rolls on those AAs.
I got an F4S from Dead Link Removed in January, and recently added a waist level DW-20 finder. Really amazingly bright standard viewfinder, though that waist level finder is a nice change when doing very low angle shots. Nice thing with the F4 is Matrix metering with non-CPU lenses, like all the old AI and AIS lenses. It also connects to much more modern flash gear than an F3, with a much higher flash sync speed for daytime fill flash.
An F3HP is nice for daytime shooting, and weighs less than an F4 (substantially less than an F4S or F4E). The flash shoe is non-standard, though Nikon makes an adapter unit so you could share a Speedlight between your F100 and an F3HP (or an SC-17). You might want to price the adapter, if you think you might use flash on an F3HP (or an F2).
An F2 is nice, though the prices are not. These are nearly collectors item status. I have also found them to be a bit clunky compared to other Nikon bodies, at least with setting shutter speeds. Nice if you find a good price on a used one, but don't buy a beat up body.
Something not mentioned yet is the FM3A. These are actually more expensive than an F4S, if you can even find one. I have the much older FM and FE, though you can also compare the FM2 and FE2. All these allow changing the viewfinder screen, except the original FM. They are light for metal (mostly) camera bodies, and reasonably well balanced with a 105mm f2.5. You might find the viewfinder brightness somewhat comparible to your F100, though not as good as an F4 (or F3).
Ciao!
Gordon Moat
Dead Link Removed