I'm sure to forget something!
Thats a huge question, lol
What are u using suppliers 4? Are they booked or r u still looking?
So the sort of stuff we looked at, and asked about, was
Price (obvs!), Carbon Footprint, Timescales, Cancellation clauses/Coolin off periods, Sustainibillity, Sources (organic local food, fairtrade, no sweatshops or animal cruelty etc).
And how quick they were 2 answer our initial messages/calls was really important - any1 who kept us waiting more than a couple of days got crossed of the list.
Your question is really broad...
What suppliers in particular are you looking at? Each one will have their own set of questions