That laptop should be more than powerful enough to run the entire game (all EPs), in fact it's overkill because the game can only use so much in the way of resources but that's not a problem. The fps issue is an anomaly, a side-effect of TS3 being an older game that lacks its own fps limiter. You should be able to run at 60 fps or right around there, it's when the actual rates go into the hundreds or thousands a fluctuate around wildly where problems happen.
You can check the fps rates you are actually getting in-game with Shift+Ctrl+C to bring up the cheats console and type fps on (enter), unless you already have a different way to do this. As you play and move the game camera around, you should see how the fps rates are behaving and whether they are staying locked in as instructed. To make the rate display go away, cheats console again and fps off (enter).
The only other thing with that configuration is that I hope you have the game installed on the larger capacity 1 TB HDD (usually the D drive) and your user Documents folder, thus including the TS user game folder, redirected to the large drive. The 128 GB smaller drive (usually C) would be for Windows and systems related programs only. If Windows and/or the games run out of available space on C then things can/will start to malfunction for that reason. But possibly you have already accounted for these things.
