The valve cover gasket and oil filter housing gasket are the two most likely culprits of your oil leak. Both are easy DIYs and no need to take the engine out. Also, when doing the valve cover gasket, I recommend replacing the vanos seals since you already have the valve cover removed.
In regards to power, the cheapest noticeable improvement would be headers. Don't expect anything more than 15 hp though. Other than that, forced induction is the only way to go but, as mentioned above, prepare your wallet.