Rushing collection of any tree because you want it can make things difficult for you and the collected tree. You may want the plants, but they (and perhaps you) may not be up to it. Unfortunately, the boxwood will have final say, regardless of your desires. They will either live or you will kill them. Rushing things will push the odds toward the latter result.
Once you dig these 20 year old plants, there will be no more to dig. You have bet the entire group on collection and aftercare skills you may not have yet. If you fail at their recovery, you will have no more to work with.
Wouldn't it be better, since there is apparently no rush to get them, to take a slower approach? Dig one now, see if you can get it to live. Collect the rest gradually, perhaps over several years. Instead of having to learn proper aftercare in a few weeks, you will have alot more time to see what works and what doesn't.
You will also find that the work involved in collecting older plants can be considerable, depending on location and the plant's health. Once you've managed to get one out of the ground, you may just be too tired to get the others
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For what it's worth, I have kept boxwood bonsai for almost 10 years. The oldest plant I've collected was a century old, 10 inch diameter, 15 foot tall boxwood dug from an old pre-Revolutionary war plantation site--it was an original landscape plant for the house, so it was pretty old.
Boxwood are tough plants that can push an enormous amount of roots after drastic root reduction. However, the older they are (20 isn't really old), the more difficult it is to get a compact root system at collection. The older roots tend to push feeder roots further out from the trunk, which can mean you have to take the "semicircle" method multi-year approach to collection, dig halfway around, backfill with bonsai soil, wait a year, then collect.
I rushed collection of that old boxwood because I had to have it. I had not been collecting plants long. I decided I had to dig it up all at once. In the end, I lost it. It took two years to croak. I thought things were going well, as it even pushed new top growth during that period. I learned later that it was running on reserves generated during its long life. It had pushed not a single new root in two years. Needless to say, it was heartbreaking...