I buy a lot of agency/freelancer portfolios and clientele.
One of the key metrics that you need to understand is intimacy.
To give you an example, if I were approached by ABC Ltd. for a sale with 200 hosting clients who only ever open a support ticket once in a while. I'd lap it up in the first instance, if the price were right.
However, if it were a smaller agency XYZ where the owners were intimate with the client - talking to them on phone frequently, doing smaller fixes for free, not just hosting them but also managing their domain and having designed their website etc. (where the client is clueless and is just trusting XYZ with everything), then the factors change completely.
You have to exercise a certain level of caution if the agency you're getting is like XYZ above. Since a change in the friendly face who speaks to them daily without their knowledge will alarm most of them and they may be in the market looking for a new developer the very next day.
Yes i understand... they are very much like XYZ...
Have you ever bought anything like that before? Have you had it go wrong with the client expecting to much etc?