If it has to be a wet system, I would use the HRM Wallstar oil boiler. They are tolerant of neglected servicing, mounted through the wall, so dont take up floor space so oil is all outside. For cooking gas may be ok for some people, but it is VERY inefficient. Are you going to offer fitted kitchens with appliances? If so I would look at induction hobs, as they are the most efficient and safest, all the user has to do is use steel or cast iron pans.
Though there are regulations for siting oil tanks, they can be varied by the BC, something not available for LPG. Oil tanks can be built any shape you want in steel. Once they are painted they need be no uglier than plastic, they have a longer life, and they don't fail catastrophically. You will find a minority of oil engineers would recomment plastic. Steel are no dearer, can be built with integral support frame, and have drip rails fitted which will prolong the life considerably. If hit, plastic may crack, steel will dent.
Please install a HW tank, not a combi. This gives the householder a backup for HW if the boiler breaks down.