Will the N,methylated phenylalanine have an easier time being reduced to an alcohol or a CH3? Or the established methods sodium borohydride, lithium, and the others still work the same?
Should be pretty much the same. You cannot reduce the COOH to CH3 with anything less than triethylborohydride, everything else will give the alcohol. You must probably also protect the nitrogen somehow (acetylation perhaps?) before halogenating the primary alcohol, or it may polymerize (the formed alkyl halide alkylating the nitrogen of another molecule).
I believe that you will get higher yields in the end if you first reduce phenylalanine to amphetamine, and first afterwards methylate your product to meth.
Also if I chose to go to the aldehyde then OH will I have a problem with polymerization as mentioned in your reply?
Exactly. Amines and aldehydes in the same pot (or in the same molecule) will invariably form imines and depending on structure, they may continue to react from there.