Hey Carling, like you I had an FTO so I know what rust is like. I lost my last car (FTO) to the dread rust so am getting mine done. Been meaning to do it for over a year since owning the car and finally getting it done this week.
My local garage is changing my front and rear subframes that have been treated with POR15 today and I will then get the rest of the undercarriage and sills treated with a specialist. They deal with landrovers so am sure they can deal with a Celica. They strip the underside and treat the rust before adding the protection (Dinitrol) and cavity wax.
Been quoted about £495 and will need car for at least half a week.
Don't really mind as I only drive about 2k a year. Not cheap but then I intend to keep the car for a good few years and paranoid after rust got my FTO!