i agree with putting a lot of thought into this decision. all three of my dogs were rescues and all had been given up by their owners. (one was given up because he got heartworms and the treatment is very expensive and sometimes fatal for the dog)
my income is small but the 'dog stuff' is part of my budget ( my former drinking money
tho i sometimes get hit with an extra expense. if your mom agrees to a small breed dog there are many of them in shelters who would love to be your best friend. just make sure you can love and care for your dog for it's whole life and wouldn't end up, thru circumstances, dumping it on someone else. that could be traumatic for the dog.
my three are my three biggest reasons to stay sober so i can give them the excellent care they need and deserve.
not to mention, our daily walks are good for all of us!