buy a7m3 and feel yourself happy. yes lens costs much maybe adapters will help here
if you need tons of used cheap lenses than propably go with nikon z / eos r
(for an example: 70-300 for sony e cost 1000 bucks and used almost same image quality 70-300 nikon g cost 30 bucks)
lumix and other systems i'm not considering to buying, too fresh.
two serious remarks: if you want to shoot street photography number one priority issue is not gear number one priority issue is safety. so i use first a7 (not s not r) for street shooting even though i have d750. cause it cost only 400 bucks used now and i dont worry much about it. with 55 1.8 i have enough light to shoot at night, even with polarizing filter (for contrast and colors).
so used cheap or new cheap is best for street photography. 5dm2 cost 300 bucks, D600/6D 400 bucks, why eben bother buying new? you can spend than money for awesome good lens, 85 1.4, for an example, btw samyang 85 1.4 manual is great and cost nothing like 120 bucks used.
D600 is best used dslr, great ISO on par with 5dm3 great dynamical range (i will use DR abbreviation
below), much better than 5dm3 and on par with 5dm4/a7m3. you just need to change shutter for oilless (till spring 2020 its free) or buy already changed. from MILCs ultimate choice i think a7 if you wnat used cheap camera
why not aps-c? because. i wrote it many times in previous threads:
"if crop factor is 2 that means that area of sensor is 4 times less than full frame 35mm
that also means that micra can do 1600 while ff 6400 on same sensor tech
for 1.5 crop number of area decreasing is 2.3x
for 1.6 crop is 2.56x"
so on canon aps-c you will get almost 3x less iso and dr, and 2x less on standart aps-c
second serious remark is: buy what you like. all camera are pretty same. better or worse. optics are pretty same. better or worse nobody cares. its all works and works great
just check iso tests on dpreview before buying because there are many traps on the market