Don’t look at it comparing the cost of the work to the value of the car, compare it to the cost of an alternative.
If you spend €1,200 - €1,500 on the car now you’ll have a car you know, in good condition.
If you don’t do the work you’ll need to find a replacement that’ll probably cost a few grand more but you won’t know the history of it.
PS. If you don’t won’t to do the work I might be interested in it as it. It could make a nice bus for winter.