Yes that serial number dates it to 1953, you were probably confusing it with an 8,000 number rather than 80,000. Which is good, since a 1953 model will use single or double perf film. Earlier cameras could only take double perf film (which is harder to source these days) but in 1952 Bolex changed their sprocket rollers to only have a single row of teeth.
Without an octameter you won't really be able to frame correctly while shooting, so it's kind of necessary. You could use the critical focusing viewer for a locked off shot (although since it views through the top lens port rather than the middle one you will have a parallax error at close distances). If the camera didn't come with an eyecup for the top critical focus viewer it might have an optic screwed in for an eye-level finder, which means you won't be able to properly view down through it. So if you attach a lens and find you can't view down through the top, you will either need an eyecup optic or an eye-level finder.
Pentax lenses should work fine with a good adapter, but bear in mind you might struggle to find a wide angle - for 16mm format you need something like a 10 to 15mm lens. A 25mm is considered the standard focal length. So you might need to get a wide angle C-mount lens.
The focal lengths on an octameter viewfinder will correspond to any lens including your Pentax ones, the octameter simply gives the view of a certain focal length on a 16mm frame (which will match what will be recorded on the film - the 16mm camera gate will crop a small rectangle out of the middle of the much larger Pentax lens image circle). The octameter works like a very simple sort of zoom, moving a magnifying element up and down a tube. The parallax correction is achieved by a mechanism that tilts the tube in towards the lens port at closer distances.