Hi Mate, welcome to the site!

I don't know much about if people will buy the car but, you see a lot of new owners popping up around that have brought their ST's 2nd hand.

doesn't seem to be an un-popular car i think a lot of people use them as practice for a faster car in the future. shouldn't have a problem selling it, longs its well looked after and cared for.

I'd probably suggest getting another car if your going to use it for motorway use, previous owner of my ST used to commute from southend to stansted airport and racked up a lot of miles in it, he added he used to fill it up on average 4 times every 2 weeks £35 each time. might make it harder to sell later with plus 50K miles on it.

you can get within reasonable average driving conditions 30-35MPG really pushing it meaning driving ridiculously slow i got 39.4MPG.

make sure it's been well looked after with full service history
make sure you check for any factory recalls or unusual things
check for modifications exhausts, air filters common modification for fiesta ST's so I've noticed.