Hi Ben
Having worked in an IT company for 2 years, I think I can better answer your question.
1. Firstly remember, every team has their own expertise (some with front-end and some with back-end development). So, hiring a single team for a complete software development may not be good idea.
2. Compare their charges with the work (hourly or sum). And always have a backup team. And do not pay the entire money upfront.
3. Have documentation on each and everything they promise and what you want.
4. See their client testimonials and check if they have already worked with a similar kind of work which you give.

Please do give me a call in case you wanna know more.... And do give me a bit of background about your issue, so that I can better be prepared