If you pick an lovely car that just need an annual MoT, insurance, tax and petrol putting in it - then MPG is obviously going to make a difference.
IMO with any 10+ year old car like a Clio 1.4 RT, youre going to have to buy wisely and with some expertise in order to keep costs as simple as that.
If youre simply looking for the cheapest motoring possible, Id buy the very best condition car you can find from a mainstream manufacturer. Few models of car can be called "cheap to run" on a universal basis for that model - by the time you get to a few years old, the care a car has received makes all the difference in terms of future cost.
In general, the Mk1 1.4 RT is a durable car and most of the normally expensive parts (e.g. electric switches etc) can be found from the breaking/spares sections. Id expect a good 35-45mpg with considerate driving and low servicing and maintenance costs with careful buying/considerate use.