I have a small collection of Olympus film cameras - all in working order. I have an OM-1, OM-2n, OM-2SP, OM-4, OM-10, OM-20, OM-30, and an OM-4. i initially purchased an OM-1, mostly to get a couple of lenses to use with my digital cameras (with adapters). The OM-1 was pristine, and it just felt good in hand - so I kinda got hooked.
I know nothing about the seller in your case, but you have received good advice about the camera from others. I use #357 batteries in my OMs. They seem to work well in all of them. I would suggest you calibrate your meter with the first roll of film - shoot something of average tonality, bracket +/- at least 2 stops, with the ISO set to whatever native ISO of your film. Then after developing, decide which setting is most pleasing/realistic to your eye, and set ISO with that film to your preferred setting in the future. I have found the light meters of most of my OMs to still be pretty much right on.
Light seals are no problem. Find them on Ebay and replace them yourself, if needed. I have found the OM lenses to be quite good. I think the 50 f1.4 to be a bit soft wide open, and actually prefer the f1.8 version. Not much more to say. They are wonderful cameras IMHO. Other good ones are the Nikon FE and FE2, The Minolta XD11, the Pentax Spotmatics, and the Canon A-1 (my personal favorite Canon - never could warm up much to Canons).
With appropriate adapters most lenses of whatever brand can be adapted to m4/3 or Sony FE digital cams in full manual mode of course. Even if adapters exist, I don't see much point in adapting film era lenses to other brands of film era cameras. Anyway, ...
Peace. ...and Best Wishes.
John