In short: The choice between an outside evaluator and doing it internally turns on independence, capacity, and what the funder requires, and sometimes a full evaluation is more than you need.
The first decision is whether to bring someone in at all. These pages cover external evaluators versus in-house staff, working with an institutional research office, and how the different kinds of evaluators compare.
An evaluation that finds a program isn't working is still useful, and there are honest ways to report it that protect the program and the funder relationship at once.
Some parts of evaluation a small team can genuinely do alone and some parts shouldn't be done alone, and knowing which is which protects both the budget and the credibility of the result.
Vetting an evaluator comes down to specific questions about fit, method, and availability. Credentials alone won't tell you who's right for the work.