as for mirrorless cams, the plus is access to pretty much ANY lens ever made, slr, rf, c mount, konica etc. the downside is that for accurate focusing it is absolutely essential to add on an external viewfinder, and then you still have no mechanical confirmation of your visual judgement, though most allow for 10x magnification
While an evf would be nice, it is certainly not required for accurate focus on the hi-rez nex lcds with 7x and 14x on nex-5, a bit different mags on 5n
A clean nex-5 body is now only about 300USD on ebay. However only the 5n can take new sony EVF and nex-7 it's built in.
The DSLRs are a can of worms. Only a fraction of MF glass can be used at infinity on any of them. I'm assuming you are planning to use legacy glass.
The nikon mount, for example seems like it would take any old nikon glass. It does not. Some lenses are fine, others will damage the camera. For the Canon its all about adapters. FD glass is useless, but other mounts will adapt: case by case.
The nex simply takes anything with the correct 20USD adapter. Sensor is APS-C so crop on 35mm is 1.5X.
As you know there are only a few full 35mm frame DSLRs and the M9, none cheaper than 2k for body.
However if you just want to MF with the native glass then you have good input here and will have to do your homework by close review study. For that I'd go to the shop and try it if you can.
For cheap access to all 35mm legacy lenses and good image quality the nex is unbeatable right now, and it's obvious from the thousands of samples, that focusing is doable, if not effortless
🙂
BTW nex does lock exposure with half press on any lens.
good luck