Finding Minimum Value Using Multiple Criterias

I have managed to make a work queue and lots of other stuff for the model, but I can't get it to take orders in the way I want it. Each order has a order number (from 1 to 100) and the orders come in almost randomly e.g. 3, 5, 11, 2, 7, etc. What I want to do is to take the smallest available order that has not been processed in.

The available orders column and processed orders look something like this:

A B C D
Time, Available, Processed, Start processing
5 2 0 2
10 0 0 0
15 0 0 0
20 0 0 0
25 5 0 0
30 0 0 0
35 0 0 0
40 0 0 0
45 4 2 4
50 0 4 0
55 7 0 5
60 6 5 6

Zero means no new orders or no processed orders. Now the Start processing column should select the smallest not processed order if previous order has been processed.

A have, for now at least, all other problems solved, but can't figure out how to get start processing column check for the smallest not processed order line. I have tried combination of Min and Max functions with If, but it soon requires too many Ifs to make any sense out of it. I also tried the Dmin function, but it wasn't up to the task becouse the model requires ~1000 lines and as Dmin only takes criterias vertically I ran out of columns . So how could find minimum from row one until current row excluding values processed so far and only checking orders available so far?