A 'proper' conversion done by a company like 'Devon' or 'Westfalia' will cost several thousand pounds (and Im not even sure they'll do them on second hand vans)
If I were you I'd do one of two things....
Either get one thats already converted - I've got a T25 and it makes surfing so much simpler, and so much nicer in winter.
Or
Get an unconverted one with wooden panelling on the inside (lots of them have got this) which will be really easy to put shelves/racks in and the floor can be laminated straight onto the floor panels.
As for the cooking, get a largeish wooden box that'll take a gas cylinder and a large camping cooker and put the cooker on the box when using it (and drill a hole for the gas tube), then you have an easy to store cooker
