They also have N'Zogbia, who really needs to perform to the standard we know he's capable of. If they can get the best out of the players they've got, I think they'll be fine up front with something like this:
------------Helenius
Weimann---N'Zogbia---Agbonlahor
As for the defence, if Luna is a good enough left back (I have no idea if he is) then I think they may be alright there. That means Baker and Vlaar could form a decent partnership where they belong at centre back, and of course Lowton is excellent on the right. Ciaran Clark could then move up to playing defensive midfield, competing for that position with Yacouba Sylla, and give them some much needed security there, and in the other CM spot they want a good passer which is really what they lack. Let's give it to Ashley Westwood for the time being:
--------Clark---Westwood
Luna---Baker---Vlaar---Lowton
------------Guzan
That's how it looks to me without any further signings, and to be honest I think it's a passable EPL side. With the Benteke and Bent money, they could get a better playmaker than what they have to offer right now with Westwood, El Ahmadi, Ireland and their other mediocre midfielders; perhaps another left back if Luna isn't all that great yet, and one more attacking threat who can perhaps also play on the right during the inevitable period when Agbonlahor becomes inexplicably useless for a few months.
Also Albrighton, Delph and Delfouneso have their moments and will continue to improve with age. Generally I think they've more-or-less got the talent and just need them to show it more often. They certainly have depth.