Hey,
I did my undergraduate at Warwick in History and I am currently doing a Masters in History (again at Warwick, since I loved it so much)
1) I did the modern stream. Many people choose Warwick for the Renaissance stream, which one is best for you is purely dependent on your academic interests. Though I am encouraged that the mandatory additional language has been dropped for the modern stream (I did French in first year), I am sure that you will need to learn Italian if you take the Renaissance stream. Having said that, there are those who had a quite flimsy grasp of Italian by the time of the Venice term in final year and they seemed to get by just fine.
If you take the Renaissance stream your module choice will be restricted and there will be mandatory modules that you will have to take. Take a look:
http://www2.warwick.ac.uk/fac/arts/history/students/options/history I also suppose this page answers your next two questions as well
2) So the point is, if you prefer the look of the Medieval World, Florence and Venice in the Renaissance, and three years of Italian over the great host of well-taught, extremely interesting (I'm not biased at all...) Modern stream modules, then go for the Renaissance. It has more glitz and glamour - you get to go to Venice, but if you prefer modern history, which I did, definitely do that. Yes it is limiting to stick to one era, and you are limited far more with the Renaissance stream. What if you do not like Renaissance history, oh you're stuck with it for three years. At least with the modern stream the module only runs for a year.
In my experience, and please do not take this as gospel, the workload only became a problem in third year. First year is actually pleasant, not as demanding as A Level. So I only have like a one hour exam, though things might have changed since then. Second year is more difficult, there is a particularly awful deadline where you have (well I had to) to hand in three long essays on the same day, but if you've planned for that you'll be fine. Third year is far more difficult, the core module Historiography is notorious and the hardest module you'll do in undergraduate History. Though leniently marked I thought.
I suppose you want more information/tips on first year. 1) Do not buy every book the course modules suggests, go the library for them, you'll end up spending too much money. There are some books that need to be bought, but only a couple. The whole point is that you do wider and in depth reading and not base your answers around the reading of a single book. If you do, buy from Amazon to save money. The uni bookshop is overpriced. 2) Since everyone will be pilfering the library make sure you get the books early, if its only a chapter, photocopy and put it back for other people to read. 3) Lectures are important, yes. Go to them if you can. However, if you miss a few, don't panic. 4) Use first year to test your essay-writing technique and get it locked down. 5) GO TO THE ESSAY FEEDBACK - i cannot overstate this enough, actually talking with your seminar tutor/person who marks the essays, and asking non-stupid questions about how to improve was so so helpful for me. A lot of the time, essay technique rather than knowledge is more crucial 6) ENJOY First Year - make friends, the work is so so manageable then, chill out a bit. You do come for the education but also the experience. 7) Do not go the other way, make sure you do try in first year. Your marks go on your transcript and employers can see them. If you get a low mark in first year, thinking its fine because it doesn't count toward degree, its not they will question it.
And probably the most important piece of advice: go to careers talks, go to workshops, law companies hold workshops specifically for first years, you need to be considering what you are going to do with your life in first year (though becomes more important in second and definitely third). This sounds preposterous I know, you haven't even done your A2 Levels but it is true. The workload is manageable, the contact hours are less than 15 hours a week, and until third year for me it was absolutely fine. Use the summers to do internships, in law, banking whatever. If you want to earn money - look at jobs in the SU or Unitemps.
Its late and its turned more into life-advice, my apologies. Truly, I never had a bad lecturer at Warwick, undergrad was a blast, the department was awesome, had the best time. Good luck. If you (or anyone else) has questions about History at Warwick message me.