What kind of league do you play in? Points, Roto, or H2H?
IMO your roster is pretty strong, especially for your first draft. I don't think you're too young, most of your young guys have at least one solid year behind them (Upton, Hanley, and Hart). Kemp is an outstanding hitter and should put up good numbers for you.
The Bentancourt situation depends on what kind of league you play in. For example, the key relief statistic in my fantasy points league is SV's, so unless you expect the reliever to soon be a closer he is wasting a roster spot. In this situation, you would only want a setup man to backup a closer on your roster to be in a tedious position, like getting Marmol to backup Wood. But if you're in a H2H or Roto league, a dominant setup man will be useful for your WHIP and ERA, and will toss in a few K's as well, and Bentencourt will be a very valuable player. In your situation I actually think you would benefit from picking up Marmol if he's available, given Wood's injury history. Marmol will be beneficial in H2H and Roto leagues, and cheap insurance in a points league.
I rank your positions as following:
SS - Hanley is among the elite.
OF - especially if Sheff stays healthy. Kemp and Hart are up and comers.
2B - Upton should put together a good year for you, and 2B is typically a thin position.
3B - I put Aramis here only because of his injury problems. And they're the crappy kind that makes him day to day, very annoying to deal with.
SP - Very solid front line in Lackey and Smoltz (if Lackey's injury isn't recurring). And Burnett, Hudson, and Lilly aren't bad at all as 3, 4, and 5
1B - Konerko is a dinosaur. He will be OK, but his days among the 1B elite are over.
C - Don't worry too much about this one, almost every team is weak here. There just aren't too many decent C's out there.
RP - Capps imploded last year. And Wood and Izzy are legitimate injury threats. This being said, this could turn out to be a very strong position for you, its just risky.
The amount of players you can start depends on your league. If players are on the DL and you have open DL spots on your roster, definitely move them over so you can pick up other guys in the meantime. I sometimes use these spots on prospects that I think may hit it big early, for example last year I grabbed Pence when he came up with a spot opened by injury.
I hope some of this helps. The best advice I could probably give is to troll around on the other Cafe forums. You'll find great information in the Injury/Player update forum, and the Buy Low/Sell High thread in the Draft/Trade forum is beneficial. Good luck!
"This guy leads the league in most offensive categories, including nose hair..."