focus on crit damage and crit rate over fort and overall attack power. As others said you can always gear level your weapons to get the attack power. A +7 forted sin can 1combo up to +15 forted all classes save pally. 40 sins can cap crit damage and still maintain 25-45 crit rate. Dpsing is always more about debuffing/crit/crit damage/multi hit than it is attack power.
Also don't buy a plvl its not worth it especially if you don't have the gold for it. Just grind yourself, you will save the gold and may even be able to farm some more gold. As in spam DT killing halfway through and resetting. And selling the blue drops.
edit; your gonna need to gear level a lot of stuff as a sin in 40. paying all your gold for one weapon to 30 is silly. Better save the 300g or spend it on A scrolls, 30 A scrolls should be enough to get your weapons to 7 or at least 1 weapon to 7. Again farm and gear level simultaneously.
While I do disagree..
^He has had more experience as an assassin in 40 than I have, so it's better that you take his advice over mine.
As for a +7 sin one-comboing +15 classes; would like to see this... Anyways I reccomended forting first, as it is the harder thing to do. Very good luck trying to get one weapon to +7! I hope the cost of repair mallets does not hinder the process. Disclaimer: Unless the sprite king loves you, you will well likely exceed 300g+ on +7 alone considering the cost of a repair mallet.
What armor are you wanting to use: (40)Sprite set, (31)Sprite set or PvP set?