So I have a 1.6TDI Octavia from 2015 (Euro 5) and I'm debating changing the timing belt / water pump.
Car currently has 80k miles.

 

Are these engines reliable? Is it worth investing money into this car? What problems can I expect in the next 25-50000 miles? Are there any things certain to go wrong up to 125,000 miles?

I will likely sell around 100/125k miles.

 

I'm only doing long journeys (100 miles plus) in the car so no short trips - I'm therefore not super worried about the DPF. 

 

Thanks in advance.

Nobody is going to dissuade you from changing your timing or water pump.  Typically (and the current thinking) - is that if it hasn't been done within 7 years and/or 120,000 miles, then it is probably due for a change.  I suspect that you would be within that range - based on age.  Have a look at the timing belt and see if there is any micro-cracking.  The water pumps from 2013-2017 were susceptible to failure, so a new water pump would not go amiss.

 

re: engine - built proof as long as the oil/filters are changed every 10k miles.

Have you owned from new or just acquired this car?

I'm almost surprised that the pump has not had to be replaced already based on the frequency that topic appears in this forum. IF it has then hopefully the belt and guides were at the same time but while that is considered a 'no-brainer' addition, obviously not guaranteed.

 

If you are a new owner of an old diesel I'd recommend you get an OBDC reader and software to confirm the status of the DPF.