Many cars are renovated without pulling the car apart to a bare shell, but it really depends what you are starting with.
You say that the body has never been restored. If it's an original UK car, that makes it very rare indeed, and could mean that it wasn't used in the winter months / bad weather in it's first few years, or it had some sort of body protection system applied when it was new. It may have escaped having its body rot away as many did when exposed to the salt and damp in our green and pleasant land!
If it is not rotten, and the shell is straight and true, then it is a perfect candidate to have rolling repairs completed in several goes over a few years.
When the professionals check over a car to see what need doing, the first thing they will do is to check the alignment of the shell. This is easier to do with a bare shell, but fairly straightforward with a rolling car, and really is a job for the professional to complete.
Quite a lot of convertibles that go into CMC for bodywork have twisted shells, usually due to poor restoration work in the 70s and 80s where too much metal was removed and the shell distorted. Or they've had a heavy crash and not been repaired properly. The only thing to do then is to strip the car down to a bare shell, cut it in half across the sills and floor, mount the front and rear bulkheads onto a jig, cut away all the rubbish and rebuild the shell virtually from scratch.
Luckily it's a rare process to have to carry out on a Coupe, but it's not unheard of.
If the car is straight and with no obvious ripples / poor accident repairs, then it's down to spotting where the rot is and developing a plan of attack. If you are fairly hands-on, you can complete a lot of the analysis work yourself.
Firstly, remove the seats, carpet, boot boards and door / side trims. That is going to show you, at least superficially, the state of the floors, the boot floor and the lower parts of the bulkhead. If you buy an endoscope and attach it to a laptop, you can then explore inside the sills, 'A' posts, 'B' posts, doors, bonnet, inside the rear wings, and the rear of the car around the hatch.
You can buy endoscopes now for less than ?15, and are a very worthwhile investment for many types of inspections....you can look inside your combustion chamber, gearbox and all sorts of exciting places!
http://www.ebay.co.uk/itm/261158772054? ... 1439.l2649
You will also need to get underneath and scrape of the underseal around all the vulnerable areas to uncover any nasties lurking there.
The plan of attack must sort out the safety bits first, if you're intending to carry on using the car. Obviously any issues with sills, floors, 'A' and 'B' posts and most importantly the pickup up point areas for the front frames and the rear subframe must be all part of phase 1. If there is a lot of rot around the front frame pickups on the bulkhead, as well as the other structural areas, then it's probably best to do the whole lot in one hit as a lot of the car has to be taken apart to sort out any issues with the bulkhead.
If the rear tubs and rear wings have a little rust in them, but are not completely rotten, they can be done as another phase, probably along with any issues around the rear panel / numberplate area.
The doors and tailgate can be another phase, and of course the bonnet yet another.
Each phase will require some sort of paintwork, and this is the cost that is wasted when you come to do the last phase, the complete bare-metal paint.
So it really is impossible to price up how much the job is going to cost until the work required, and the method of completing the work, is quantified.
As you can see from above, there is a lot of work to be done just to quantify what needs to be done, and if you just drive the car in, you should budget on spending around 2 to 3 days labour to provide that quantification. If it's not done up front, then things will inevitably be uncovered as the work progresses which could upset the approach you take and, inevitably, the cost.