Not familiar with what your specific duties will entail - Intern and all, but it sounds like you could be hauling a lot of gear, doing multi-days... (out for weeks at a time?) - or not... who knows.
I would ask your superiors for a bit more beta - then ask around and see what others used last season. In short, find out what worked best in the past..
Would be a bit presumptuous to recommend anything without hearing more...
3-day trips, you could get by on 60 liters, but suspect 70+ would fit the bill better.
Maybe you will be carrying other "group gear" on top of your own - then maybe a load-hauler is required.
Be aware that most the current ultra-light packs seen today are usually carried by experienced hikers who forgo a lot of creature comforts in order to lighten their load...
All that being said, and not knowing any specifics about you - (weight/height/experience), and realizing you have a limited budget, I would first go to a local REI and get measured correctly. Then go on line (
Geartrade, eBay), and buy a
used (correct size) Gregory Shasta, Palisade (or whatever) for under $100...Maybe a pound heavy but proven load carriers that will last a season or two without breaking the bank and self-destructing on the trail. Next season, if you come back, you will know better. Spend any extra coin on a good sleeping bag instead.