How big of a deal is it to take a few walk-ons? Does my football team need 50 solid players or will it be fine with only 45?

You want to fill every scholarship. The walk-ons are awful and you don't get to carry over any recruiting money.

You want to try to fill all schols and have the full 50 players be solid. Even though depth doesn't mean as much as it used to there are still injuries and they will develop and turn into good players

+1....walk ons suck

If you are going to take some walk ons, I wouldnt want to take more than 3 if it can be helped.  At the end of the season you can cut up to three players.  You dont want to be stuck with more walk ons than you can cut the following season if at all possible.  But filling scholarships to be able to redshirt players for the future would be better in my opinion. 
Since your taking over a team that is all sim recruits to begin with, signing players would be very important.  You can offer starts, playing time to help convince them. The walk ons will be far worse than the sim recruits currently on the team most likely.