I distinctly remember the mini-campaign my own group played back in 2E, still referred to as "The Munchkin Campaign". We started with 3'750'000 exps, and free reign on items. We had a level 20 arcanist, a level 23 druid or so, a level 28 or so rogue, and a level 23 or so fighter with vorpal weapons and a homebrew cerebral knight (psionic class). As long as the group was roleplaying and hanging out in Sigil it worked pretty well, but combats were ... a wash. To properly challenge such a high level party would have taken too much preparation time to make it worthwile. Anotehr problem was the differing preparation levels among the players. I had my arcanist written out with 20 to 40 pages of background, and went through the spell lists with a fine comb (all supplements I could get my hands on), while the druid player forgot that druids could heal...
I would suggest to start at a medium level and work your way up, possibly accelerate the exp rewards, but to play at least one adventure per level in order to get familiar with the capabilities of the PCs. (And to build up engaging backstories. IME, combat gets dull fast, but plots keep the players coming, but your experience may vary.)