Not all partners are equal and I worked with a good one before. First, depending on your problem complexity you may or you may not need a partner. If the problem is small -to low medium and if hiring another developer and a BA is enough then do it in house as it is more engaging to the business users and leadership.
If the problem is complex and needs long time, find a partner who is specialized in such kinds of problems. If you have a customer engagement problem, don't choose a partner focused on F/O. The benefit of the partner is that you and them will agree on the requirements before starting work and they are accountable for delivering what was agreed on. Make sure that you flush out every possible details and don't leave a requirements that is open for interpretation.
Also, Having a good developer on your side if the Partner is doing the work is very important. with his better knowledge in the system and the organization, he can guide the partner in the right direction when designing the new system. Also, he is a good support/developer candidate for the system after the work is done with the partner.
Another advice if you want to work with a partner. You have to make sure that your organization users are engaged with the new system from the first release. Don't wait until the last moment to test what the partner has built, if you do that, this is a recipe for failure based on my experience with many projects. Have a team of end users who have some technical sense and let them test every time the partner deploys the solution, track the bugs and let the partner know. At the end, these will be super users and your system will be super good :)
Last advice, you and the partner will agree on a very specific amount of work for some tons of money, this means, if in the middle, you want to do something new or change an existing functionality, most partners will deal with this as a separate change requests which is understandable. To reduce the amount of those change requests, spend some good time with the business coming up with the requirements of the new system and gold-plate them carefully before hiring a partner to find the requirements for you. This will save you money later, believe me.
