- what you want
- what you actually need
- the priorities within those lists
- what level of service / availablity
- your budget (and how realistic it is in the context of the previous points).
Most ppl new to such efforts - and even many of the experienced - have unicorn ideals in mind, and a fast food budget.
p.s. Also be ready and willing to sharpen your communications. Assuming they'll know all their is to know about your effort isn't going to get you far. Unless you're able to hire a mind reader ;)