The WO doublets (of which the ZS81 is one) are pretty well-regarded. A bit more false color than a triplet, but they're not bad. I had the ZS61 for about a year and was quite happy with it.
Yes, the only difference is aperture/focal length (and size/weight). They're all FPL-53 doublets. They require different flatteners/reducers.
What size filter you use will depend on the size of your imaging sensor. For sensors smaller than APS-C, you can usually use 1.25" filters (but it depends on where you put them in the imaging train!). For APS-C and above, 31mm or 36mm or 48mm filters will be required -- again, it depends on the size of your imaging sensor and where you put the filters in the optical train.
One nice thing about the WO scopes -- very good build quality. Smooth focusers, powder-coated tubes, nice anodizing. I have a GT71 and love it.