Erhahon was probably Lincoln's best player last season and a combination of an initial lack of fitness added to a couple of injury niggles and a loss of form means he hasn't had a proper run in the team and hasn't hit the levels he did at Lincoln.
A fit and on-form Erhahon would give the team far more balance as we're short of left footed players willing to stick a foot in. I agree that he's probably not the best footballer, but does have that will to win. In fact, he's very ungainly and looks almost awkward when playing, but there's definitely a player there.
As for Johnston, I think we've seen the best of him already. Yeah, he's committed, but isn't tall, isn't quick and isn't that physical. He does read the game fairly well, but like most defenders in L1, his decision making and reaction time is poor. That said, he's probably no worse than Toal and only marginally worse than Forino.