for the HD gun,
keep the standard trigger
get a short, sturdy stock like the magpul CTR or MOE
grips are really totally personal preference. i have used several and lean toward the magpul MIAD (see above) for most stuff but occasionally the ambi ergo grip.
don't waste any money on things like anti-walk pins, accu-wedges, etc.
although not "lower parts"
you'll want a good flash suppressor (e.g. vortex) and not a brake. and you'll want a good light. the surefire with vtac mount is a good reasonably priced combo ($54 for surefire light and mount)
for the 50bmg upper, you either want a fixed stock or a collapsible that locks into place and won't collapse when you shoot it (potential scope-eye). the magpul prs would be great for this BUT you want to make sure the raised cheek piece doesn't get in the way. i doubt you're using a CH on a 50 though so that's probably not an issue.
another stock that would be super-sturdy to take a look at is the ace socom
lots of triggers would be appropriate for the 50. i'd probably go with a 2-stage. RRA's are pretty cheap. if you're looking for super-crisp and proportionally pricey, geissele's are in stock in several places.