Folders and (heavy) handlebar bags don't always work so well IMHO. Having that weight mounted so high on the bike messes with steering and center of gravity.
I agree with fietsbob, Brompton is probably best here for a simple reason: Having the mounting clamp on the headtube instead of the bars means steering is not loaded by the extra weight in the bag. Downside is that it's all probably over your budget. If you don't get a Brommie, though, then we're back to the question of where a heavy bag fits on a folder.
My solution on the non-Brommie bikes is a Detours bag that attaches to the seatpost. Then the other side - paring down to the bare minimum for me: lightweight Nikon DSLR, 10.5 Nikkor fisheye (incredibly tiny lens!), 30/1.4 Siggy, 50/1.8G Nikkor.
Having this weight on the back of the bike induces a slight springy effect, but I find this much better than a h-bag on my non-Brommie folders. YMMV.