Let's do some economics here...
The actual cost of having a running cal lab is the same as using an external cal lab, excluding the shipping costs. This is assuming you are using it at ~100%, not much idle time, but if you have much idle time you can sell that time calibrating for a 3rd party. This is basic economics, if you have any doubts get back to the books.
The decision then is about if the interest (plus reduced shipping costs) of having it in house are high enough to justify the investment. Plus maybe the extra benefits as fastest time response, custom jobs, etc. Without forgetting the headaches it would bring, as the need of special trained staff, higher standard calibrations, etc.
Then, economically is all about the duty cycle of the fully working cal lab, the shipping costs and the interests of the initial investment. You are here asking, particularly, for that last point. Which does depend on the specific equipment. To give an example 4.5 digit DMM isn't specific enough, most 4.5 digit DMM could be calibrated with an 34401A or equivalent, but not all of them as some may have an specially low range or wired function that even a 6.5 digit DMM wouldn't be able to deal with. Of that will depend if you could get away with a reasonable prized 6.5 digit or you have to go even higher. With that the external supplies and transfer standards. I don't know particular cases of scopes but I guess similar things could happen. Other factor to consider is the amount of work, to know the level of automation needed, I guess that with the info you gave plus the timing of the cals, an experienced person in the field should give a first approach of what you need.
You would need to do in one day, 2 DMM, 1 Diff probe and 1/2 scope to have the lab working 200 days for once a year cal for everybody. That leaves space to send out the gear to an external cal lab twice a year. As you would be using the lab a lot you can't work directly on your standards, but you need transfers to be calibrated in house maybe after 90 days if not shorter. Then you are probably good with one 6.5 digit DMM and one capable of calibrating it which would be the reference one. If it happens that there is a few 4.5 digit DMM as the one I told you do have the better one to deal with it. This is just a guess and only for the DMM with my limited understanding of the topic.
JS