If doing 5,000 miles a year you shouldn’t have a problem with the diesel, maybe take it for a run once a month but the economy won’t be any better round town as a petrol engine uses way more fuel until it’s warmed up anyway and on a run the diesel will beat it hands down, the missus has a DS4 1.6hdi and only does 2,000 miles a year, every other weekend we nip out for a trip to an outlet village 30 miles away in it and it hasn’t had any dpf issues at all and even though she only does 4 miles a day to work and back her average last year was 57.6mpg.
On a side note the dealership can’t ask for the advance payment up front under any circumstances and if they have you should report them straight away to Motability, if it was for extras that you chose then that’s between you and them on how much you leave as a deposit but I would always make sure it’s returnable.