I have seen the ME and MF engine codes used for both engines with hydraulic lifters and engines with solid lifters which makes some iterations of the same block code NOT interchangeable. That was a really bad move for VW to make.
The MF engine code was used for turbo-diesels and has piston cooling oil jets installed in the block. The ME engine code was used for non-turbo engines and does not have the piston cooling oil jets. From what I have seen, the ME blocks have the pad machined flat where the jets go, but do not have the holes machined for the jets to be installed. It is possible to have those holes machined in the ME block and install the jets. You could potentially transfer them from the TD block you are replacing. With the squirters installed, the blocks would be functionally the same. I have a solid lifter ME block that I have been planning on turning into an MF block by adding the jets. I will probably make a jig for hole placement and afterward might rent it out for others who would like to do the same.