I had a 97 Fiat Cinquecento (91k on the clock) for 5 years (adored the low petrol usage,easy parking, 1.1 engine so quite nippy) My husband hated it because of the safety aspect (no air-bags, so small that a minor collision could have meant me wearing the engine!) Finally bit the bullet on 1st July and bought a Yaris - can't understand why I resisted so long - although I do miss the sun-roof, esp. after the weather this week!!
It really depends on your other outlays elefantfresh - mortgage, credit union loan, credi card bills etc. IMO you can't put a price on your life or your family's lives. On the other hand, if you're happy with the Fiesta and wouldn't have problems if you were car-less for a few days, in the event of repairs etc. no point shelling out €15-20K. Fiestas are good runners, not rustbuckets and hold their value quite well. Even at 10 yrs old, the low mileage should mean its worth around €2000-2500 privately or trade-in.
BTW main dealer allowed me €1700 on my little car, (a year younger but 20k miles more) so maybe do a bit of digging to see what you'd get trade-in and then decide. Nissan are doing a 90's deal and will allow you 2-3K depending on what model you buy. IMO can't fault Toyota from the safety aspect, (9 air bags in the Yaris model I have- although to be fair I'm prejudiced as a Carina E saved my husbands live twice!), good value for money, 3 yr free-recovery(even for something as minor as a puncture!) 3 yr service warranty and 12 yr anti-corrosion. Sorry waffling now!! Love the new car!! Hope this has been helpful and it all works out for you, Amanda.