millage is irelevent its how its been driven and looked after, cars that have been driven hard are not a problem, its the ones that have been abused to watch out for. you can usually tell with a quick drive.
as for 172 vs 1.2, i get 40 miles LESS to a FULL tank of petrol that i got with my 1.2 16v, so its not a problem, the 172 has buckets more torque than the 1.2 so doesnt need to be driven as hard around town.
tax on 1.2 16v was £115 ish i think last time i checked and 172 just over £200 so again not THAT much in it.
insurance will be your biggest issue depending on age.
12k service intervals compaired to 18k on the 1.2 16v