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FWIW, I can see the Cassini division quite clearly with my 80mm Triplet refractor at around 120x magnification (4mm EP) and above but, the image itself is still quite small... A triplet is probably well out of his budget but, if he can deal with some false colour on brighter objects, an 80mm Achro & a lightweight mount would probably come within his budget & weight considerations.
My 5" Vixen Newt would offer similar resolving power, would possibly meet his budget second hand & is quite light but, it's bulkier than an 80 - 100mm refractor & as you say, might present some collimation challenges
Maybe a small Mak Cassegrain? 127 - 150mm, fairly long focal length in a reasonably compact, lightweight package perhaps?? Might be doable secondhand within his budget on a modest mount... Should be able to resolve Saturns rings with relative ease but, again, image size isn't going to be huge...
My 2 cents...
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