I did this for around a year. There used to be a forum for it that everyone posted the ones from can's/packets that didn't need a purchase as well.
Remember anything easy will be done. Your money is in setting a process up for bulk printing the back of postcards and other stuff people can't be arsed to do. I took 1000's of the free postcards you see advertised in pubs and had a word merge job going. Every friday just printed 100's of them off and merged the comp address. The ones that wanted a tie breaker or answer question were a bit of pain in the ass.
Cost of 2nd class stamps wasn't pretty either.
I won quite a few things but nothing useful. Toilet seat with drawing pins in, dog gravy, 7 different colour hair dyes, two 1 carat uncut emeralds (not worth the card they arrived taped to!) and a mini keyring torch thing. My best win was a years supply of either Tiger or Cobra beer, I forget which. ****** A1 win that... until I tried to claim and they were going through a merger or something so they wouldn't honour it. Spend loads of time on it and solictor just advised I drop it. F all I could do
BTW you are going to have to be very lucky in 2.5 months. The forum used to have example figures of how many people entered certain comps and in some case it was many many hundreds of thousands for one prize so you are playing the numbers game when they are stacked against you. You are gonna need a lot longer than that to start reaping the rewards.