As Bill says (and of course you already know from your ALPY experience), the guider is important. With a bit of ingenuity and machining skills though it should be possible to incorporate the guider and mirror slit from the ALPY into your Littrow design making the grating holder and rotating mechanism the only difficult part. (This is a critical part and has to be designed and manufactured well if unwanted movement is to be a voided)
(I do rather enjoy modifying existing equipment where most of the hard work has already been done and I have vague plans to take the components of an ALPY and turn them into a universal setup with a range of grisms/filters mounted in a filter wheel between the collimator and camera lens, similar to that often seen on professional telescopes.)
Alternatively the mirror slit used in the L200 is still available from the manufacturer Ovio and is significantly cheaper than the Shelyak ones. I will see if I can find the reference.