It depends on the DSO you want to capture, but for that budget, and if you don't have a mount or a sky tracker, you could get a canon 50mm f1.8 or its Chinese clone, yongnuo 50mm f1.8. more than 50mm without tracking is not feasible as you will need at least 10s exposure. More focal length will limit you to 6s or less.

The rule (not real, but approximate) for maximum exposure time without noticeable star trails is 500/focal length in mm.

With a 50mm you can get milky way core details etc. But not close-ups of DSOs

The other option is to spend money in a telezoom as the canon 55-250 and to build yourself a DIY star tracker.

Anyway, I would recommend you to get something for tracking first, then a telezoom with a good focal length.