You don’t give any indication of the age of the 307, or whether this has the original 2 litre or the later 1.6 (both are designated Hdi 110). The 2 litre unit in my Citroen (same engine) has a change interval of 100k miles. It’s on it’s third belt now having reached 205k.

Both these engines should return a genuine 50mpg without trying, so engine life and economy are not reasons to change the vehicle.

But if it’s got to go, I wouldn’t bother changing the belt first. You would be unlikely to recover all the cost in the increased PX value. If you sell privately, just set the price accordingly.

If you like the A3, a mechanically identical VW Golf will be cheaper, and an equivalent Skoda or SEAT even more so.

A diesel will give you similar or better fuel economy, but unless your annual mileage is over about 15,000, consider a petrol: it may use more fuel but it'll cost you less in the first place, even buying used.