Mass-effect – the maximum possible N7 level

A newly created Shepard starts at Level 1.

Early levels go by very quickly however. For example, you'll be level 3 by the time you get off of Earth. Additionally, your opponents are scaled to your level throughout the game, so once you get past the first few levels, (where a lack of available skill points hampers your tactical options), you shouldn't have any particular problems being a low level character. It might make reaching level 60 in one play-through more than a bit problematic however.

Based on how it worked in Mass Effect 2 and personal experience in ME3, enemies scale to levels, regardless of how the levels were acquired. As such, the answer to your question is yes and no. Compared to an unimported character at the same place in the campaign, it will be more difficult, but compared to an unimported character of the same level, it should be about the same difficulty.

Edit: Now with science!

I played through the first 15 minutes of the campaign twice, once with my imported ME2 character and then with a brand new character. Below is a spreadsheet with the amount of body shots I counted before the enemy died. All shots are from the starting pistol. Note that these are wild approximations - it's hard to count with friendlies firing as well, and I likely landed a few headshots without realizing it. Note: Both characters attained levels over the course of testing. No skills were spent on either, and only the character level at the start of each area is listed

As hypothesized, enemies on Earth were roughly the same difficulty (it was made as a basic tutorial). Once I reached Mars, though, the enemies seemed to take more body shots when Shepard was at a higher level.