As a team with D prestige, how good of a prospect can I reasonably expect to sign? I feel like to really turn things around I need to aim higher and this is upcoming class is crucial as I have six scholarships.

IIRC, people with overall ratings under 550(ish) will talk to you immediately and people under 600 will possibly give you a backup plan message. I have four recruits left from my endeavors recruiting with D- prestige, and checking their ratings histories, they had initial ratings of 480, 545, 547, and 580. 

I also think overall ranking of the player and postional ranking come into play.  On one D prestige team, I got backup messages from everyone ranked 60-95 at their position (also worth noting this was likely a low D prestige).  The second time I recruited with D prestige (and I thought I should have been D+, so probably the higher side of D) I was able to immediately talk to kids ranked 60-100 at their positions.  

Only way to know for sure is to send a phone call to find out where you stand.  If you want, you can immediately send a few HVs or CVs to the kid and see if you get considered.  If the kid is out of reach they'll just reject them and you'll get your money back.