I won’t answer for all that fall towards the foreign policy orientation in the GOP but…
First, since the President has stronger powers in relation to war, diplomacy, and negotiating trade, I weight those highest when selecting President. That makes me quite a bit different than most of the electorate.
I generally take a realpolitik approach to issues. Use all means available, (diplomacy, economic, aid packages, and military force) to pursue national interests after looking at the costs and benefits of pursuing a given plan. A lot of the neo-con influence that’s become more dominant among my party’s foreign policy wing tends to ignore second and third order effects of dealing with an initial problem. That skews us away from making optimum decisions since we’re ignoring possible benefits, and especially costs, from the intervention.
Trump is ignorant, apparently willfully, without a diplomatic bone in his body. He’s actively proposing trade protectionism which empties some of the economic tools to pursue foreign policy out of his toolbox.
Clinton is okay for me. She has experience in the trenches with major geopolitcal issues. She understands the methods and knows many current key leaders. Putting together the coalition for Operation Odyssey Dawn (aka Libya) was a nice piece of diplomacy. She still doesn’t seem to have figured out why I ranted at my TV for 20 minutes while proposing “Operation Failed Nation” as the name. IMO her answers with respect to Libya during the campaign show either a lack of understanding of the value of stable nations with respect to our national interests or a failure to see the risks of ending an authoritarian regime without being willing to commit troops to provide security while we try and build a stable government to replace it. Well it could be both. 
That worries me. Like the neo-cons she seems to want to apply the military force hammer without understanding all the likely effects. She’s no Elder Bush but she’s a reasonably good fit for my important issue.
To me it seems like he mostly backs into effective positions after the politically acceptable, but unlikely to succeed options, give way to reality. The best argument for him that he’s just doing that to keep from losing political will before he ends up pivoting to what works after the populace has stopped paying attention. It’s probably a whole different thread for why I wish Clinton had gotten the nomination 8 years ago instead of him.
In general, it’s pretty important because their current, and most likely going forward, goals are focusing their populace in nationalist intervention in their historic security zone. That produces instability for our NATO allies even without them being directly targeted. (For example, there’s a pretty big refugee issue due to fighting in the Donbass region of Ukraine even if the news only covers Syrian refugees.)
Agenda is a hard question. Things are going on in all those areas and Africa (which gets a lot of attention). The American public doesn’t pay attention to most of it so it doesn’t make the news or get broad coverage in political discussions. The recently failed peace initiative in Columbia is a prime example. Most probably didn’t pay attention. Fewer would have known there was an active and long term engagement supporting the government’s counterinsurgency efforts going back to shortly after 9-11. I’ve been involved with training National Guardsmen to deploy to the Horn of Africa which included engineer units to dig wells.
Those kind of things don’t make major campaign agendas. It’s hard daily work. Things also change significantly between campaigns. They don’t resonate with voters and it’s hard to draw a lot of distinctions in a 30 second sound bite unless it’s along an interventionist vs non-interventionist line.
I, personally, tend to focus less on agenda specifics for the things that make the campaign coverage. I assess them for judgement and predispositions.
That was Trump level of idiocy IMO. Don’t make a threat when, realistically, there’s no meaningful course of action available to make it credible. He could have at least saved a little face and kept a little bit of teeth in future threats by feeding some Tomahawks into something early. He dithered long enough that Russia managed to diplomatically take even that off the table.
Worse, his campaign demonstrates that he doesn’t listen to experts that can at least help him muddle through the complexities.
I voted for Barker but I didn’t vote Trump !