Quote: so should I get a SLR
well, it sounds like you'd like to, so yes.
Quote: why or why not
see above.
Quote: which one. why that one.
depends whether you use your lenses manually or AF currently more often.
the MZ-5, MZ-7 style of bodies will get you in the door most easily. You can always manually focus regardless of body.
The trade-off is that the earlier non-AF bodies generally have a much nicer/brighter viewfinder which is somewhat of a revelation if you've not used one before to focus with.
Quote: AF or manual. why or why not.
see above.
Quote: and what about printing costs
still reasonable at many labs for basic 4x6 prints, a few include that in the base cost, as they used to.
for starters, i recommend a place like Darkroom.com largely because they bundle basics scans into the base price, and their web-layout is easy for small orders of just a few rolls or less.
---------- Post added 11-27-19 at 10:01 AM ----------
Addendum, without going overboard:
A) If you'd like a camera that is
most analogous to your current digital bodies and just want to try a bit of film with your current lens sets, the Z-1 or PZ-1 would be a good fit. Very affordable and its DNA is the same as what you already shoot with. You'll feel at least mostly at home. (You can also shoot your lens that do not have an aperture control ring with these bodies because aperture can be set with the camera itself, just like your DSLRs.)
B) If you'd like to branch out into older manual bodies, I'd recommend the KX. It is the simplest out there whilst still offering most niceties a shooter might want, and is often the same price (sometimes less) as the sought-after K1000 which is very similar in form/function, but lacks a self-timer, DoF preview and has a more basic meter to name a few differences. The KX can be found fairly reasonably in price.