Best additive to soil for a large garden?

We'll finlay be planting our vege garden this spring but it is going to be a big one. I was wondering what is the best soil additive that won't break the bank that I can probaly buy at a nursery or Lowes, by the bag? I know compost is problay the best, but does that come in 50lb bags? I'll problay need a lot since it's going to be a farely big garden, but I don't need a dump truck load..lol.. Also how do I check ph levels, not the soil, but what level that certain plant needs. I'll be planting a lot of different veges so what if the ph levels don't come close for all the different plants, can I still plant them?