If you have the experience, and know whats involved, all you need is the money. I think 250k - 300k + vat is more realist, to fit out say a 2000sq unit. Also if you go with centra they charge an additional ~10% of the fit out to cover the cost of advising you! Ring them up, they will meet you within a day or two, and answer all your question better than anyone here!!
The GM for a convenience shop would be in the range of 22% - 27%. Depending on product mix.