I disagree with the above re the 1600, I have one, it was my first car, still use it as my daily car now.
I actually find mine very economical (The book figure is 40MPG), I do around 20 miles a day plus any social driving in addition.
Also most petrol engines best shifting practice is around 2-2,500 RPM imo.
As said though the common place to watch for rust is in the corners by the wing mirrors and also the rear wheel arches if they haven't been looked after properly.
Some also have reported drop link problems.
The ST170 is ok but personally I would rather have a 225 if you don't mind paying a little extra, both of which are thirsty.
Also be aware the 170 is a pie in the sky so I have heard, in the real world they never make this power, not alot less say circa 160 but is isn't what the badge claims!
The 1600 is the most common and is the best if wanting the best compromise of economy and performance.
Only Focus with a chrome tailgate strip is the Ghia model!
The 1.8 gives a little more grunt , whilst the 2L is the range topper of the MK'1's (Obviously excluding the hot models both of which are 2L anyway)
To conclude in my ownership I have never had one problem!
To Bailes I see yours is an LX, as you correctly say the seats are not brilliant, they are better in the Zetec or as you say the ST170, it's not uncommon for people to change them for this reason, as you plan to yourself.
They really are good cars (Even if I do say so myself) imo one of the best cars Ford make nowadays.
Martin