Locks and Shelves can be modified or added easily. I recommend avoiding looking for a more expensive commercial freezer. They have a tendency to be less reliable and a waste of money.

You specifically DO want frost free (as you mentioned). Temperature variations are (within reason) insignificant for photograph collections (and most museum collections for that matter). The key is the temperature, not the variation. And the slight variations that occur with the auto defrost are 1) fairly small, and 2) probably not even felt by the negatives within the boxes in any case. And frost-free saves you from having too defrost manually from time to time and allow the negatives to age at room temperature while you do.

Also, consider adding a thermometer that will notify you if the temperature goes above a set-point. I believe Hobo and Lascar loggers, among others, have such features.

Feel free to contact me off-list if you have any further questions.

best,

Gawain