RE: 318tds to 325tds swap
To my knowledge should be a straight swap providing you have all bits. Afaik you'll need to keep the obc to retain cruise control as it will not work with out it.
RE: 318tds to 325tds swap
I am not too sure on diesel models, but 325tds is a 6 cylinder car. Which means it runs a bigger gearbox and bigger differential.
At least that's how it works on petrol models.
For example, if you wanted to drop 2.5 petrol engine instead of 1.8 - the whole engine + gearbox + driveshaft + rear end would need to be changed.