I don't know about Lukaku but Babacar works very well with lots of players like i know a guy who has him in his Arsenal Squad and he works very well with Walcott. He is a poacher so you'll need a guy who holds the ball long enough for Babacar to run past the defence then for a through ball, one on one on keeper. Maybe some sort of tall strong striker? Like a target man?
Christian Benitez is very cheap (4M) and scores a lot, so if you bought him you'd still have some left over, also, try not to use Monthly installments too often as it will wreck your finances
As said above, Christian Beneitez is a good bargain striker that is quite good.
If not, you could try some sort of lone striker up front (Babacar) him as a poacher and have a formation of as such, 4-5-1 or 4-1-4-1 or some sort and get Eden Hazard as the playmaker. Either LW or AMC which works well for him.