Choosing which instruments to record live

You will constantly have to face budget cuts. If they tell you beforehand, you'll be fine most of the time. Where it gets really annoying is if they do after you've written lots of music and then you are told that they need to cut the budget for recordings, which means: Fewer players.

When deciding which instruments to record live and which have to done with samples, there are two big questions:
1) What is your kind of music? If you feature a solo instrument quite heavily, you should record that one live under all circumstances. For ensembles, think about using one or two real instruments per type and making it bigger with samples.
2) Which instruments can you replicate will with samples. In most cases, percussion can be done with samples, if necessary. One exception would be the timpani, they tend to sound better with real players. A piano, if only used for support and adding rhythm, can also be sampled pretty well. I'd tend to retain the brass section; if your piece has lots of soaring brass, it might actually be cheaper to hire real players then to spend ages programming samples.

Also never forget that any piece will only sound as good as its arrangement - if your piece is not arranged very good, even the best live players cannot make it sound good. So double check your arrangements before recording :)