I didn't noticed that the OP is located outside the US.

But it still stand to reason that it would be less expensive to buy a road bike, than it will end up costing to buy the Trek 750, and replace everythin on the bike except for the frame and fork. After all, isn't that what the OP is talking about doing? OP is talking about replacing the seat, wheels, shifters, and how the gears are worn (front and replace, hinting at replacement. What does that leave?

I'm aware of the cost of buying a bike and replacing everything except for the frame. I've done it several times myself (most recently on a Trek 1100, which came with a 3 X 7-speed freewheel, and Suntour components, and now sport all Shimano 3 X 8 drivetrain and new wheels). I just got in a 94 Trek 1400 frame and fork, and plan to install my choice of Shimano 8-speed components.

The Trek 750 is already a good flat bar utility bike. It sounds like the one the OP bought needs some serious overhauling. I hope he got a good price on it, because it sounds like he's headed for a 100% rebuild.