If you are interested in building a compact Littrow design using a camera lens you might find this design of Christian Buil’s interesting if you have not come across it already.
You need a pretty good lens for this short focal length fast f ratio design to work well. He talks about the choice of lens, particularly the need for it to be compact enough to be able to mount the grating close enough to avoid vignetting. He used a Nikon lens (210 Euro in 2002) but perhaps you could try the 50mm f1.8 Olympus lens which was used with the famous Olympus OM1 SLR camera and can be picked up second hand at a good price.
For testing on a simple scope, how about a cheap small aperture Newtonian? Off axis performance is poor but you only need a good image on axis for slit spectroscopy and because there is no chromatic aberrations will be much better than a cheap refractor or camera lens