I'd turn it into a hobby room, a home office or a guest room. If I really was being decadent, I'd use it for storage - a"junk" room.
I liked your idea of a study, but in my case, I'd just have a comfy reading chair with a good light and a small table to hold a beverage next to it. Perhaps a TV or stereo system (CD/DVD/DVR AUDIO/VIDEO EQUIPMENT) and maybe even a small desk for my laptop and printer. I would love to just "kick back" in that sort of uncluttered space.
I would see if the room had good natural light, and if so I might put some plants near the window. Maybe I'd get an aquarium or just keep it simple and have a terrarium. Some people might like to put exercise equipment in there.
Since money is an issue, you could have a "daybed" which is like a futon or a couch that can become bed for guests. But for book cases or other furniture, depending on your tastes, go to garage or rummage sales, or thrift shops like Goodwill, and buy simple bookcases or chairs or even inexpensive antiques. Buy only what you like, because these may stay with you awhile. I still have wooden chairs I bought when I was 20-Something for $2.00 each 35 years ago. I painted them colorful colors, and now they are part of the family!
I detest IKEA RTA (ready-to-assemble) particle board stuff, becuase it is made cheaply and may not last as long as a classic oak antique chair, but everyone has their tastes & style preferences.
FYI -Particle board stuff can give off formaldyhyde fumes. Not good. Particleboard is made of wood shavings held together with a formaldehyde-based resin that outgasses. You can see the wood shavings throughout the board on both the flat side and the cut side and it has an odor--strong when new. Formaldehyde offgasses from particleboard until there is no more formaldehyde to offgas. It is very intense when the particleboard is new and decreases over time. I read that the half life of formaldehyde in particleboard is three to six years, but can still contribute to indoor air pollution 12 to 24 years after installation (Spectrum--Newsletter of the Society Promoting Environmental Conservation, Vol 26, No. 1). This seems reasonable to me. How long any given installation takes to full outgas depends on the conditions. More heat will make it outgas faster.
Good Luck! It's nice to have you own place all to yourself. ENJOY it while you can!