I currently use the Samsung galaxy tab s 10.5 but it may be quite a expensive for what your needs are.
I do agree with what Acsel says. However, I dont find battery life a major problem with my tablet. Its when your doing CPU intensive tasks like games or lots of multitasking + background apps, that really drains the battery. Just sitting with it on and occasional scrolling should be fine (depending on tablet). I couldnt give you a number of hours though, but 6 should be just about fine (just reading that is). However, if it does run flat, whack it on fast charge and go make yourself a cup of tea and im sure you will be able to finish what your doing before the end of the day with enough charge.
Look at tablets specs and any brightness above 252 cd/m2 I think will be fine for outdoor use. Maybe google it.
With multitasking, android would be your best bet. You will have access to the entire google play store, with lots of apps available for word processing and productivity. I just use google docs so all my notes are in the cloud and can be accessed on any device at any time (useful when typing notes in lectures then using library computers to add to them). I dont recommend a kindle fire. My mum has one and I find them quite slow and frustrating. I wouldn't recommend hacking anything as I think that may void any warranty.
Also, I would by separate keyboard for whatever device you chose. Typing on a tablet takes some getting us to, and writing notes in bulk will be a challenge. I have a cheap (£17) Bluetooth keyboard that attached to my tablet and its pretty easy just to type away for hours on end to in lectures. also gives you the ability to "Alt - tab" to another app, like a pdf document, making multi tasking easier. Some tablets even offer split screen abilities to have two apps open at once to save you switching back and forth.
Personally, I recommend buying an android tablet. Maybe spend a little more then you planned to have one a bit more future proof and better functionality. I use my tablet religiously at uni for all notes in lectures and seminars on the go, everything syncs straight into the cloud so I can go home and have it ready on my laptop (very recommend). I can read and manage and pdfs or Ebooks very easily. You will also use it a lot outside of academia whether its just watching a film while cooking or just browsing the internet in bed. If your a girl then it will fit in a handbag a lot easier then a laptop.
However, each to their own. Excuse my dyslexia. Check reviews before buying anything to make sure its what you want. hope this helps.