Welcome!
A few members will tell you that Lurtz isn't worth it BUT I think he’s a mandatory leader for an uruk scout force. Although, I mainly play for fun and do not play competitively
I've never really thought about combining uruk scouts with upgraded marauders but it sounds like things could get confusing when trying to differentiate between the two types. You could simply say that all the uruk scouts with bows are normal uruk scouts (and to declare that all those with hand weapons are marauders) but see my next point:
A common tip that I do agree with is to avoid giving uruks regular bows. You’re keeping a STR4 model out of combat and with the new hobbit rules, shooting (especially for evil) isn't very effective. With speed on your side + bouncing around terrain I don’t think you need to include any bow armed models.
Shields are a must for warg riders and marauders.
Nice touch with the ringwraith on horse.
Good luck
(
Technically it's an Isengard/Mordor force hehe)