What you're experiencing is a mismatch in what you all expect the actual game to be. As such, a boon will likely not make up for the confusion — at best it will be inexplicably ineffective at altering the players' choices, and at worst it will exacerbate the problem.

Different games, same name

You see roleplay and adventure in a believable world as the essential activity of the game, and perhaps the one player who kitted their character out with gear reasonable to an explorer is also on your wavelength.

The players who are buying only combat gear do not see that as the essential activity. To them, the game that lies ahead of them in their imaginations is one largely composed of combat and mechanical puzzles — mechanical in the sense of game mechanics, not levers and pulleys in the game world. Effectively building their character for the (expected) challenges of the combat-focused game that they are anticipating is of utmost importance, else they will fail, and the failure will be entirely their fault for making a "bad" character. They're naturally avoiding that at all costs, which includes wasting money on stuff that they won't need — or if they will need it, will appear conveniently within the adventure nearby where it needs to be used.

You can perhaps see why these two sets of beliefs about the future activity of the game could both arise from "let's play D&D," and why they are going to have many points of friction with each other. You are likely just seeing the very tiniest tip of the iceberg here, in terms of the ways these players will conflict with each others' and your own ideas of what the fundamental point of play is. To use someone else's analogy, it's as if you've sat down and said "let's play cards," and you have Go Fish in mind while others are expecting five-card stud poker. Without discussing beforehand what you're actually doing and what the point of it all is, you can easily get partway into a game before it starts to break down under the tensions of people trying to play the game "right," according to how they expect it to work.

A minor boon is too small a tool for this problem

A minor boon seems like a good idea — positive reinforcement, minor enough not to make lacking it effectively a punishment, and an encouragement for the players to alter how they play to get the little bit of extra benefit.

Except it's not going to work that way.

• The player who earns it from the beginning doesn't need it, because they already want to play in that way. It's just free stuff for nothing to them.
• The players who you most want to encourage different behaviour in will not change, because getting the tiny boon at the expense of fatally compromising their character's mechanical quality is unacceptable. On the off chance that getting the boon can be done without compromising their character at all, they'll do it — without changing their play in any other ways.
• The players who don't get the benefit it will feel like they're on the wrong end of DM favouritism, and will resent that they're taunted with an offer of a "false" choice that they would never accept.

The fundamental problem is that you're all trying to play a different game, and a small benefit that makes sense within the game you're trying to play is not going to work as expected in the game they're trying to play. The root of the problem is deeper and not going to be fixed by addressing a minor symptom.

You need to agree on what you're sitting down to do

Are you playing a game where emotive performance is paramount? Where clever problem-solving using the tools at your disposal is key? Where skilled leveraging of the interactions of combat rules leads to success?

These are all different goals. They're not incompatible, but not all caring about the same goal is incompatible.

To fix this, you have to sit down and have a conversation about what "playing D&D" means to each of you, and find out if you have any common ground. Yeah, this may seem like taking a game too seriously. The alternative is to just forge ahead without worrying about it — maybe things will work out organically somehow, maybe they won't, and taking that gamble is totally an option.

If you want to take a more measured approach, or if the game group ending messily is something you want to avoid, you have to get on the same page before you continue much farther down this road and the players get more invested in their personal views of how the game "should" be.

Handily, there is The Same Page Tool for this purpose. Basically it's a checklist of things that are currently all assumed by you and your players, but which are mutually exclusive ways of and purposes for playing a roleplaying game in the first place. Using the tool to explain what you're assuming (it's not a survey for everyone to fill out individually, you shouldn't have more than one copy on the table), gives you a starting point for explaining things about the game you intend to run for them, things that probably you don't even realise needs explaining or even knew existed as points to differ on.

Fill out your personal vision of the Same Page Tool, and bring it to your group. See if they agree, where they disagree, and have a conversation about that. Maybe you can all adjust and compromise — it's likely that you can. Maybe not, and you'll have to figure out what that means for you as a group, then. Regardless, knowledge is power. It's way better than groping around in the dark and having unhappy play experiences, because roleplaying is supposed to be enjoyed!

There is no absolute way to tell, no spell that says "he's level 10/10 HD." You have to go with context clues and observation instead. That leads us to a two-part answer.

DM Description and Observation

Observe more closely and the DM should be more forthcoming with details. In many cases, higher level NPCs/monsters look different - think World of Warcraft, you can tell a high level person because of their crazy pimp looking gear and purple energy coming off them and stuff. Maybe he has ioun stones whizzing about his head, etc. Judicious use of detect magic, detect evil, etc. can tell you strength of auras. A DM should also think through setting up context clues per How can DMs effectively telegraph specific dangers in D&D? - like maybe someone powerful comes to mess with him and gets disintegrated while he's chatting amiably with the PCs.

Many high level characters are obvious - a gnome in a robe of the archmagi riding a dinosaur with all kinds of magical protection glimmering around him is obviously bad news. If all your gear is platinum and adamantite it means you're bad ass enough to keep a hold of it from random bandits and/or adventurers. Your DM should be conveying this detail, as long as you pause to take a look.

In some cases (monsters, undead) skill use may let you get a better read on what it is you're facing. In 3.X this was sadly routine (I will roll and you will tell me exactly what this is), in 5e this isn't as guaranteed a method of knowing exactly what's up and there's more DM discretion, but at least asking for a roll and getting hints might help.

It definitely sounds like your DM needs to be thinking through this more and giving more context. "Hey this dead guy looks a lot like that statue of Archmage Xulibraz back at the Acadamae..."

Play Smart

In some cases, a high level person isn't going to look/detect different from a low level person. This should be more rare unless your DM is a dink, though there are exceptions. A high level monk in rags looks like anyone else and a king might have super pimp gear despite being level 1 just because of daddy's money. Now, if you see the monk fight it should be like watching Jet Li and you should get the clear impression he could snap your neck and isn't some random yokel. But this is where the onus shifts to you. If you decide to just roll up on someone without knowing much about them, there's a good chance you're going to get owned one day. You have to be smart.

Observe folks. Ideally, use time prior to the actual encounter. If you know about them way ahead of time then there's divination spells or sage research or "asking around on the street." See if you can observe them fight someone or cast a spell or use a skill to where you would see "holy crap they are about 500% better at that than I am." Have a backup plan. If you just kick down doors and attack people, 1 in 10 times you're going to bite off more than you can chew and you'll need a fallback. That floating thing could be a beholder or a gas spore. That undead could be a zombie or a lich. Knowing that there is that range of foes, it's up to you to decide whether you want to be all reckless and maybe die one day or whether you want to be super cautious with everything or where you want to fall in between - just like real life.

Conclusion

You have just learned a valuable lesson, and learned it without dying! Congratulations. The world is dangerous and it's not always neatly labeled or level appropriate. Let this inspire you to think about how to suss out your foes both from afar and when in direct contact with them. It'll keep you alive more than a fistful of plusses.