Is it appropriate for poor students to save money to buy SLR?

Support the landlord. If you are really interested in photography, buy it.

Photography itself is a profound art, which is worth your efforts and financial resources to pursue.

Besides, you have done a lot of homework, and I believe this road will not make you regret it.

The SLR itself is more valuable after buying it, and the lens is more valuable than the fuselage.

Like the D90 you bought, the price itself has been reduced to a minimum. Working with Nikon's cost-effective D-head is a good start.

I am a college student myself. I did my homework for more than half a year and finally bought the D7000. Up to now, the shutter has exceeded 6000, leaving many beautiful memories and some "blockbusters", which can feel the mood of the landlord. Since it is a beloved thing and an elegant and just hobby, it is really a flower that I have thought about many times and still like as always. Go and realize it.