Best mods on a puma are lower it a bit and a strut brace and bigger brakes, and then go for the performance things like exhaust and things like that as they are suprisingly quick standard for what they are.
Economy wise it depends on how you drive it really if you drive them, I can get easily get 150 miles to £30 if I drive around semi sensibly but if you gun it everywhere it drops down to about 115 so still not to bad, for the fun you have.