If your current car has high mileage then you'd have to be careful that you don't end up spending lots of money on the running gear that won't have been replaced.
You may be able to find another ghia fiesta mk4 for a decent price, there's enough about and you may pay a couple of hundred more for one with a decent engine but it'd save you the hassle and you could sell your old one as is. Although I guess you can be sure of your car's history and you may not be of another one. I don't think that the bells and whistles make that much difference to the selling price at this sort of age, it's more about low-mileage/FSH.
I had a 1998 MK4 1.4 ghiaX. I loved it but pt/exed it earlier this year when it got to 95k and began to show signs of needing a new clutch etc, it was well maintained too, had air con ew, cd, electric height adjust seat and the like but I wouldn't have got more than =A31000 privately because of the mileage, that's why I got rid then before I shelled out more cash and the car went over 100k and was worth zilch. I pt/ex'ed it for =A31500 on a 2004 seat ibiza.