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The Saxon 1.25" achro 2x barlow will probably do you for starters but in the longer run you might want to get something better...(?? hard to say for sure because Saxon have three different models out). Bit of dust or small marks on the lenses won't affect the view in significant way. Collimation is by far the most important thing to get right especially for mid-to-high magnifications.
The "cross shape" you mention is likely normal: it sounds like diffraction spikes from the "spider" (the four steel vanes that hold your secondary mirror in place).
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