I too shall follow this with interest.
I built myself a ~ £300 3D printer a couple of years ago. One tricky issue I came across was the accuracy of the printing. I never really achieved better than 0.2mm accuracy. So, mechanical components generally needed a lot of cleaning up with a file before they were usable. I didn’t have much luck fine tuning the calibration, but perhaps I was just incompetent. 🙂
As regards mechanical strength, I think you almost certainly want to be using ABS. I found that my PLA prints couldn’t be left in tension for more than about a few weeks (depending on thickness) without snapping. This improved somewhat if I made the infill 100% solid, but at the expense of using lots of plastic and taking ages to print. I gather that PLA degrades and deforms particularly fast if exposed to moisture, so in a dew-laden observatory, I’d certainly favour ABS’s chances!
There even seemed to be differences between suppliers. The plastic I bought from RepRapPro (sadly now defunct) seemed noticeably stronger than what I got from various other suppliers.
While 3D printers are a lot of fun to play with and they’re great prototyping tools, I think making a precision spectrograph is quite ambitious!