Hi Peter,
welcome to the forum
Values of 1st gen RX7's are all over the place at the moment... some cars sell sight unseen on ebay for sums that they clearly aren't worth and recently there has been a trend for average (condition 2) cars to attain high hammer prices mainly from dealers who then retail them at ludicrously inflated prices and they tend to stick around for ever.
As a rough guide, nice high end condition 2 to lower end condition 1 cars are fetching between £5000 and £8000..... cars over that figure are either mint exceptional cars ( I can't think of any that have sold recently off hand) or highly modified bespoke cars which justify the uplift...... insurance values rarely reflect the true market value of a car.
It was widely accepted back in the day that an Elford Turbo car was worth a grand above the price of a stock factory car but this was back when they were newish and all of a similar condition and as stated above, condition is the most critical aspect these days.... forget history, mileage, spec, turbo or n/a, and colour...... buy primarily on the condition of the body/ monocoque. Sure, history etc is nice to have but if the shell is bad then good history /low mileage is no use whatsoever. There is a guide to checking bodywork and the typical problem areas somewhere on the forum.
Look for the Ebook on the forum... it's a free download of road tests and buyers guides and has some good info on buying these cars and particularly the stuff to look out for in recent years.
1st gen Rx7's were actually very well built and actually fair better than many other cars of the era but as the youngest one is now 32 years old they are all going to have some corrosion somewhere.... obviously the way they have been stored and how many winters they have endured on Uk salt laden roads will have had an impact on the bodywork.
Generally, they hide their corrosion well and unless you know where to look, it's easy to think a car is very tidy when underneath and in the box sections it can be corroded. Once the outer panels start to rust you can be sure there are plenty of hidden problems.
Be especially wary of cars that have had a full respray unless there is photographic proof that it was done right.... proper bare metal/glass out resprays cost £££££ and there are a good few cars out there that have had a quick blow over to hide the rust.
Both of the Elfords mentioned in the above posts, the one sold at ACA and the black one in Scotland have both been resprayed.
Specific advice on buying an Elford... they are a standard car with a standard 12a engine to which the factory induction was stripped off and a simple draw through turbo set up added. the only thing to go wrong with the Elford setup is the turbo which costs £200 to get rebuilt. It should have a solenoid on top of the engine to retard the ignition under boost... some 'owner converted' cars are missing this.... very risky! Everything else is as the standard car.
If it has the bodykit check behind it for rust and also carefully check the Elford front spoiler for previous damage/repairs ( they are extremely fragile and almost impossible to get hold of these days.)
Being a turbo it will put more stress on the engine so a compression test is a good idea... anything less than 6's is coming up for a rebuild and 12a rebuilds are getting expensive due to a lack of parts. ( I have an Elford turbo engine sitting under my bench that it just isn't economically viable to rebuild).
The only other thing I'd add is that these cars, like all classic cars, are happier when they are regularly used... cars that have been off the road for years need plenty of recommissioning and replacement parts like master cylinders, brake rebuilds, coolant and fuel system overhauls etc.
My advice is to buy on condition alone.... if it was a toss up between a really nice standard car or an average Elford I'd buy the standard car every time... it's much easier and cheaper in the long run to make it go faster than it is to restore a tired car.... I could convert a standard car to an Elford Turbo over a weekend ( assuming I had the kit... I think there's one on ebay atm for £400?) .... it would take many more hours than that to do a rolling restoration on a tired example.