If you're going to challenge you want at least Gold, as that gives you Economy Plus at purchase and lounge access on the international itineraries (though you may not have access to partner lounges when on a challenge). The E+ alone would be worth well in excess of $400 given your flight plans. With Silver, you're limited to E+ at check-in, no lounge access. You would get one free bag on UA and Premier check-in, which can be meaningful if you need to check-in at the counter.

UA challenges are limited to once every 5 years - your call if it's worth burning that now.