You'd be better with a 200tdi. I have a 2.5 N/A myself (basically 2.5td without the turbo) and it's a good running solid engine, but a 200tdi has more power and it'll run a bit quicker. A 2.5td and 200tdi will both get between 25 and 28 mpg but a 2.5td will take to vegetable oil and bio fuel better than a 200tdi.
When it comes to working on the engines and getting parts, a 2.5td is much easier to work on and a lot more simpler to fix. A 200tdi has a bit more compressed in and a load of turbo pipes to get in your way. Parts are just as available for the 2.5td as they are for the 200tdi.
As for a 300tdi, a newer engine still, but even harder to work on with regards to awkward parts and positions. It's a touch quicker, has a bit more torque but wont be as good on the fuel consumption.
A 2.5td should suit you well, but you'll have to mind not to thrash it. It's an early turbo engine and they weren't really designed to hande a turbo block, but they do, and they can handle it, but if you push it too hard you might bend a pushrod or a crank shaft. A 200tdi is probably going to be your best bet if you want to pull up hills, tow a trailer and generally keep up with traffic as soon as they've left the traffic lights.
Good Luck!
-Pos