Have to disagree with anyone who says you "have to" do xx miles a day before a diesel is worthwhile. The price of petrol and diesel is almost on par now and a diesel is anything upwards of 25% more efficient (35mpg in a petrol on a good run, 50+mpg from a diesel).
The point is not the cost of the fuel, its the additional cost of the car. You need to be doing quite a few miles a year to recoup the difference in purchase price with the money saved by
lower fuel consumption.
For what it's worth, I think the OP is making the correct decision by going for the diesel engine. I've owned pretty much every variant of the Mk4 series TDi and the 1.8T, the TDIs are good engines for what you're looking for with a few exceptions.
I'd advise you quite strongly to stay away from the ARL engine in the 150bhp FR TDi and
Cupra TDi. They have well documented problems with the camshaft lobes and followers and will set you back over £1k to repair when/if they go south.
The ASZ 130bhp engine in the Leon 1.9TDi 130 SE is a very good engine without the problems of it's 150bhp sister, but it's still a PD and has that stupid combined pump/injector system.
In my experience and in my opinion, the very best of the Golf MkIV platform TDIs is the AHF or ASV engine code in the 110bhp Leon 1.9TDI SE/SX. It is FAR FAR more reliable than the later 130/150 PD TDi engines, it's more economical by at least 5mpg on average, it's cheaper to maintain as it doesn't require PD friction modifiers in it's oil, and a basic remap alone will give you 140-150bhp and just as much torque as the PDs. The 110bhp TDi is the only diesel engine I'd have from the
Mk1 Leon range. It is the best for a daily driver by quite a large margin.