The cost should not be greatly different from a gas installation, except for the oil storage tank and any differences in the price of the boiler (not enormous, I believe - call Plumb Center for a quote of the two, use an equal rating to obtain the comparison). Counting the number of radiators is only the very roughest of guides (a dumbed-down notion for "the public", or consumer, who are/is often treated as a complete idiot), their respective sizes and ease of feeding pipes to them, plus the amount/number of controls you decide on are the real indicators. I would recommend that you avoid microbore piping, size your radiators to heat the space quickly (i.e. high rather than low output) and use thermostatic valves throughout, room thermostat(s) and zoned heating - but make sure you are also well insulated as soon as possible. Then you should be able to enjoy long overdue comforts at a lower cost than fuelling "fires" and freezing most of the time. Best of luck.