When your FTO arrived from Japan, possibly up to 15 years ago now, it will have been almost completely free of serious underbody corrosion. The FTO was never intended to withstand the rigours of European winters and as a result underbody protection is almost non-existent. If your FTO hasn't been undersealed since it arrived in this country then you may well encounter corrosion problems sufficiently serious to fail the MOT test. Certainly this applies to the north of the country where use of salt on the roads in winter is much more commonplace.
Apart from the obvious such as metal brake and fuel pipes, the rear chassis rails are particularly vulnerable to rust, as you can see from the picture below, together with both front inner wings. The very worst example we have seen suffered from serious corrosion in all 3 areas, together with a leaky fuel tank and wafer thin rear suspension arms. Although it looked reasonably tidy up top apart from slightly raggy rear wheel arches, this particular FTO was a virtual scrap yard underneath. It was deemed uneconomic to repair by the owner and it was eventually sold for spares.
The image below is of an unprotected FTO that had been in this country for just over 2 years. Most of the corrosion was just superficial, and not at the MOT failure stage but if left it would certainly be a source of future problems. The picture shows the offside rear chassis rail and it can be clearly seen that rust has started to invade the spot welded seams. The second picture shows the inboard view of the rear suspension and once again you can clearly see the effects of only 2 years of UK corrosion on the suspension components.
A vehicle will fail and MOT test if serious corrosion exists within 30cm of a prescribed area - essentially any load bearing point used by brakes, suspension, steering, seats and seat belts. Any corrosion determined by the MOT tester to seriously affect the structural integrity or operating systems of the vehicle is classed as a fail. Brake, fuel pipes and even badly rusted suspension components are relatively easily replaced, but repairs to the chassis and around suspension mounting areas will require welding and if the repairs are extensive then this is not going to be a cheap job, as you can see from the pictures above and below.
If you know your FTO hasn't been undersealed and you have had no MOT problems to date, then either 1. you have a massive chassis-protecting oil leak under the car or 2. you are probably on borrowed time. It's almost never too late to have the car undersealed or preferably Waxoyled, but the longer you leave it, the greater the potential cost. On older or more neglected examples, getting rid of the loose rust, essential before the application of any treatment, is a real nightmare of a job - and an expensive one at that.