Welcome to the forums!!!
When you say poster size, just how large are you considering? I still have my K100 and also a K5 (the original or "classic"). I have printed with my K5 a 20" x 30" and the detail is absolutely stunning (landscapes and architecture). For product photography, I would agree that the K-01 is certainly worth considering. You are going to use the rear screen anyway, so lack of a viewfinder should not be a problem.
What focal length are you using that appeals to your product best - especially when you see the 18-55 is not cutting it? If the 50 macro shows way too much detail (and the right focal length), and the 18-55 not enough, something like an older A 50mm f1.7 should probably do very well.
The more resolution you have (thinking K3), the better the lens you are going to need to actually realize - make use of the resolution. Even to really be able to see the difference between the K5/K5II and the K5IIs (which has the Anti-Aliasing filter removed resulting in an 8% increase in resolution) you need the best glass to be able to see the difference.
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The differences between the K5/K5II/K5IIs/K-01/K30/K50 (all using the 16MP sensor) and the K3 is primary resolution, in your product photography scenario. Better AF, AF tracking moving objects, light metering, white balance are all pretty much a moot point since you are using lighting in a studio setting, and using manual focusing. The larger difference is going to be acquiring the glass you are going to want.
I would use your 18-55 to determine the focal length that works best for your situation, then start looking / asking about the best lenses for that focal length. There are a ton of videos on Youtube that show various lighting techniques for various situations. Also, the size of the resulting product - how large a print - how large a poster, etc. With that, there is a lot of talent here (professional photographers too), that can help and make suggestions.