MGO is horrible! Headshots are OP, lag is particularly bad and the controls are ridiculous. And it had quite a lot of level up, too.
I think the best examples for what the OP is proposing are Killzone 3 and Battlefield Bad Company 2. The grind in these games were not big. And you could get a favorite full loadout pretty soon (specially in KZ3).
The most important thing is for beginners to be able to get their favorite loadout on full power quite soon, so that there isn't a difference in raw power between a beginner and a guy who's been playing for a longer while. It's also important to let the player choose what skill he wants to buy first. If the skills are supposedly balance, there is no sense in reserving some of them to very high levels.
That being said, I think it's OK to still need to buy your skills and stuff, but as something that would add versatility to your choices; not as something that would add raw power. KZ3 got pretty close to these principles. You could get any full loadout you wanted, quite soon; but to get all loadouts it took quite a while.
So, in detail, my suggestion is:
1- Let players start with a few options for boosters, weapons, items, etc. Perhaps two options for each. These free early options should be chosen based on being particularly intuitive, relatively easy to use and being particularly different from one another. Those pre-order bonuses would add to these options, giving a little extra versatility to beginners. Although a beginner wouldn't have a lot of choice about his loadout, he would have as much raw power as an old player. And this is fair.
2- And this brings us to the point: no skill upgrades! Skill upgrades are an outdated and unfair concept, which have some charm and are still being used. Like QTEs are finally going away, I hope leveled skills will fade away, eventually. They are a dumb mechanic, specially when the leveling up requires the players to use the skill in some specific way during the matches. That makes the player focus on these specific uses, instead of playing for the objetives and cooperating with his team.
3- When going to level 2, a player gets to buy a new option (from among ANY of the options) in one of the categories, say... booster slot 1. When getting to level 3, he can get a new option at booster slot 2. At level 4, a new weapon. At 5, a new item. At 6, a new option at booster slot 1. And so on...So, in this example, you would be able to get access to your full prefered loadout quite soon, since the first levels are gained very quickly. But you would still want to level up more, to get more options from which to choose and diversify your playstyle.
4- You can mix in some purely aesthetic stuff in some levels. And for the aesthetic stufff, I wouldn't mind having some stuff that would be either very expensive, or only available at higher levels, cause it would allow the oldd guys to show off, while not hurting the balance of the game.