Celtic Heroes

Yee save that knowledge elixer and 10k for later.

If you want a smooth gameplay for the lower levels try buy a heroic amulet of rejuvenation or his new equivalant now for 170k ish. It regens 20 hp and energy and gives you some stat and armour boosts.

To be honest 1 knowledge elixer is not that much. To give you an idea how many you need to reach end-game, should take you like 400-700 knowledge elixers to hit 220. If you can afford it buy as many as you can. They go for around 4k ea from other players on most servers.
So to answer your question, it depends what you call high level. Most people start lixing around level 70-80 because it's starting to get a grind around those levels. Below those levels you level fairly fast without elixers.

So to summarise what items you should be looking for in game:
-heroic amulet of rejuvenation (neck, regens hp and energy and gives stat boosts)
-super experience elixers(aka xps/sks, You gain 4x more xp from every kill)
-sigils(optional, not necessary, regen your health or energy out of combat)
-Event gear that boosts your stats and skill damage or skill points boost