Engraving affects re-sale value. It might also confuse an insurance company if you tried to make an insurance claim for a camera with someone else's number engraved on it. Conversion to newer batteries (only necessary for the OM-1 variants) will also affect resale value.
If you are considering an OM-1 that is neither an OM-1 MD or an OM-1n, then it is really old!
When the first OM-1 came out, it had room for the motor drive connections, but it was necessary to send the camera back to Olympus to have the necessary installation and modification performed. Olympus was amazed by how many requests they received for that work, so they quickly decided (1975?) to include that facility in all of their new cameras, and designate those bodies as an MD model. That decision was made very early on, and relatively few plain OM bodies were made.
The OM-1 meters are pretty reliable - once battery issues are sorted out. There are more problems with deteriorated foam. A good CLA will deal with both (assuming no damage from the foam.
For both cameras, with only a little bit of practice, you can tell by feel what aperture and shutter speed are set. Having both the aperture and shutter speed dials concentric around the lens really helps with this.
If it helps you, I'll give you my perspective.
I began shooting an OM-1 (no MD) in 1974. I've had and used several OM bodies since then, including an OM-1 MD and an OM-1n. I'm now down to an OM-2s (bought new in the 1980s) and an OM-2n (bought used from eBay in the 2000s) and a couple of OM-Gs bought for tiny prices on eBay.
I no longer have a purely manual OM body, but I would be happy to use one again (I use the meter in manual at least half the time).
I do shoot flash, and make good use of the TTl flash options.
And I don't use motor drives!
My suggestion would be to choose the first good option available. If you like OM bodies (of course you will ) then you will want another one anyways.
Hope this helps.