As for brand of belts, I typically go with Gates since they make OEM belts in a lot of instances.
As for tensioner, I am of the replace it mindset. Namely due to age in your instance. As for which brand of tensioner, OEM. While you have an Explorer, I have an '03 Focus (same manufacturer and year range) and had problems out of non-OEM tensioners (rattling noise from brand new aftermarket tensioners) when I replaced it. I ended up going back with a new OEM tensioner and problem was solved. No more rattling. I don't know if it would be a similar experience for you with an aftermarket tensioner, but I'm making mention of it just as a heads up.
As for idler pulley, no real recommendation on brands other than use a good one. I used a Gates idler pulley for my '03 Focus. No problems yet and it's been about 6-7 years now.